Thirty-six domestic cats received 100 iu hCG (i.m.) on day 1, 2 or 3 of a natural, behavioural oestrus. Twenty-two anoestrous cats were injected with 150 iu pregnant mares' serum gonadotrophin (PMSG; i.m.) followed 84 h later by 100 iu hCG. Twenty-four to 26 h after hCG, all cats were examined laparoscopically to determine the number of ovarian follicles and to recover follicular eggs. Mature eggs were cultured with conspecific spermatozoa and examined 30 h later for cleavage. Within the natural oestrus group, cats on day 1 produced fewer (P < 0.05) follicles and total eggs than females on day 2 or 3, and 88.9% of the day 1 eggs were degenerate or immature and unsuitable for in vitro fertilization (IVF). Although only 54.5% of the cats in the PMSG/hCG group exhibited overt oestrus, mean (± sem) numbers of follicles (9.7 ± 0.8) and oocytes recovered (8.7 ± 0.8) were at least twofold greater (P < 0.001) than those measured in the natural oestrus group (3.7 ± 0.6; 3.4 ± 0.6, respectively) or subgroups on day 2 (3.7 ± 0.4; 3.3 ± 0.4) and day 3 (5.7 ± 0.8; 5.3 ± 0.8). Overall, the proportion of eggs cleaving in vitro was similar (P > 0.05) between the natural oestrus group (48.3%) and the PMSG/hCG group (50.9%), but the latter group produced more than twice the number of embryos per donor. Embryo quality was unaffected (P > 0.05) by day of hormone treatment, and more than 80% of all two-cell embryos were rated good-to-excellent quality. In summary, there is a temporal relationship between day of sexual receptivity and follicular egg viability in the domestic cat: eggs on the first day of oestrus are not optimally responsive to an LH-like stimulus. There is also no evidence that PMSG/hCG treatment compromises egg quality or subsequent fertilizability in vitro. On the contrary, use of these gonadotrophins markedly improves overall IVF efficiency by increasing the total number of high quality embryos produced.