Treatment of pregnant cats with FSH on Days 33--37 and hCG on Days 38 and 39 induced development of vesicular follicles (mean 9.3 follicles/cat), ovulation (mean 3.4 corpora lutea/cat) and behavioural oestrus (5/7 cats). In the gonadotrophin-treated females, oestradiol-17 beta concentrations gradually increased but serum progesterone levels remained constant although in saline-treated females mean serum oestradiol-17 beta concentrations remained basal and progesterone concentrations gradually declined. The results indicated that (1) the feline ovary and related mechanisms for inducing sexual receptivity were not refractory to exogenous gonadotrophic stimulation during mid-gestation and (2) hCG administered after serial injections of FSH during pregnancy may potentiate ovarian oestradiol-17 beta secretion.