The aim of this study was to determine whether the FecB gene influenced some aspects of fetal development in sheep. Carrier (BB/B+) and non-carrier (++) female fetuses were recovered at specific times of gestation, namely, days 40, 55, 75, 90, 95 and 135. The results showed that the FecB gene influenced litter size, body weight and ovarian development during fetal life. The mean litter sizes were larger (P < 0.05) and body weights were lighter (P < 0.05) at most gestational ages in BB/B+ than in ++ fetuses. Morphometric studies of the ovary showed that the development of the BB/B+ ovaries was retarded: the ++ genotype had more oogonia at day 40 (P < 0.01), more germ cells entering meiosis at day 55, more primordial follicles developing at days 75, 90 and 95 (P < 0.05), a greater loss of germ cells by atresia at day 90 (P < 0.01) and more growing follicles (P < 0.01) and more antral follicles (P < 0.05) at day 135. Differences between the BB/B+ and ++ genotypes in the plasma concentrations of immunoreactive (i) inhibin, i-FSH, bioactive (b)-FSH or (i)-LH were not apparent at any age except for i-LH at day 75 (BB/B+ > ++; P < 0.05). Likewise no differences were noted in the contents of ovarian or adrenal oestradiol or i-inhibin except for i-inhibin in the adrenal at day 75 (++ > BB/B+, P < 0.01). No differences between the genotypes were noted in the i-inhibin contents of the mesonephros at day 40. In mid- to late but not early gestation (i.e. days 40 and 55) significant correlations (i.e. P < 0.05) were noted between litter size and body weight at days 75, 90 and 135, and between litter size and ovary weight, ovary volume, adrenal weight and pituitary weight at day 135. To eliminate the effect of litter size, equal numbers of BB/B+ and ++ embryos were transferred to respective recipient ewes, and fetuses were recovered at the equivalent of days 40 and 90 of gestation. The results showed that the genotypic difference in fetal body weight at day 40 (++ > BB, P < 0.001) and in number of oogonia at day 90 (++ > BB/B+, P < 0.05) were independent of litter size.(ABSTRACT TRUNCATED AT 250 WORDS)