In this thesis, a rabbit line selected for growth rate (line R) is reproductively characterised, assessing the losses throughout gestation and potential causes. The document is structured in two sections: (i) consisting of two experiments, where losses in different stage of gestation and progesterone, 17?-oestradiol and IGF-I serum levels at different times of gestation are determined, as well as the influence of embryonic and maternal genotype in these losses, and (ii) consisting of three experiments, where relative expression of candidate genes related with embryonic development and implantation are studied. Line R presented a low response to ovulation induction, as well as high implantational (31%), foetal (40%) and perinatal (15%) losses. In contrast, ovulation, fertility and embryonic development rates up 48 hours were similar to those observed in other rabbit lines. Gestational losses may be due to low 17?-oestradiol levels at 12 and 24 days of gestation, which involved a decrease of progesterone levels at 24 days. Meanwhile, high IGF-I levels at 12 days of gestation may indicate metabolic alterations in this line with influence on gestational losses. In the second experiment, embryonic and maternal genotype effects in losses were studied. Results showed that embryonic survival (up to 14 days of gestation) and foetal placental weight depend on two genotypes, whereas foetal survival (at 25 days of gestation) and foetal weight depend on embryonic genotype. Foetal survival at 25 days of gestation was higher for embryos from line A in independently of recipient genotype. On the other hand, foetal weight and foetal placental weight were higher for line R. In this experiment, female weight throughout gestation was monitored. Females of line A presented higher increase of weight between days 14 and 25 of gestation than females from line R. However, weight at 25 days of gestation was similar in both lines. In the second section, relative gene expression of Oct4, VEGF, erbB3 and TGF-?2 genes in rabbit embryos at different stages of development prior to implantation was analysed to assess the right time for preimplantational valuation. So, there were differences in Oct4, VEGF and TGF-?2 expression, with the expression of VEGF and TGF-?2 increasing at sixth day of gestation, whereas Oct4 expression decreased at day four. Since Oct4 is a transcript associated with pluripotency and VEGF and TGF-?2 are related with necessary phenomena for implantation such as neovascularisation or cellular proliferation, results of this experiment suggest that from the fifth day of gestation, Oct4 expression decreases in rabbit embryos as an intrinsic mechanism to allow implantation. Reduction of Oct4 transcript expression observed at 5 and 6 days, in contrast to over-expression of VEGF and TGF-?2, could be related with expression of genes related to angiogenic effects in maternal endometrium. Moreover, in this experiment expression of erbB3 receptor was detected at four, five and six days of development, although there were no differences in expression at different stages of embryonic development, probably because its function is focused on differentiation and maturation of the blastocyst. Since the choice of a suitable reference gene is essential to obtain reliable results on gene relative expression and these rabbit lines presented different phenotypes, in the fourth experiment, influence of line genotype in stability of reference gene was studied. Stability of two commonly reference genes used in rabbit embryos, namely GAPDH and H2afz, and the geometric average of both, were analysed using three different algorithms. Then, in order to normalise relative gene expression data obtained in embryos from both lines, the genotype was analysed to the Oct4, VEGF, TGF-?2, erbB3 and Inf-?. Results showed that normalisation with GAPDH or with geometric average of GAPDH and H2afz is more appropriate when studying relative gene expression in rabbit embryos regardless of the genotype. Thus, in the last experiment of this thesis, relative expression of Oct4, erbB3, TGF-?2, VEGF and Ifn-? genes was analysed in endometrium and embryos of two lines at day six of gestation, and relative expression of genes related to pregnancy and the IGF signalling system, such as IGF-I, IGF-II and their receptors at the same time of gestation were also analysed. Results only showed significant differences in gene expression of IGF-II receptor, which presented lower expression in endometrium and embryos of line R. In conclusion, line R, selected by growth rate, showed elevated losses throughout gestation that could be related to low serum levels of 17?-oestradiol and progesterone, and higher levels of IGF-I. Moreover, genic expression of factors related with the implantation process does not seem to provide information on causes of losses, except for lower expression of IGF-IIR receptor in embryos and endometrial tissue at day six. Higher serum levels of IGF-I and lower expression of IGF-IIR may indicate metabolic alterations in line R.