In order to study the effects of recombinant human luteinizing hormone (LH) and recombinant follicle stimulating hormone (FSH) on the pre-ovulatory follicle, immature pregnant mare's serum gonadotrophin-treated rats were used. Follicles were isolated by dissection and incubated for 4 h in medium 199 containing different concentrations of the gonadotrophins. In control medium the follicle-enclosed oocytes remained arrested in prophase I, but in the presence of recombinant human LH or recombinant human FSH, a concentration-dependent increase in the proportion of oocytes which underwent germinal vesicle breakdown (GVB) was seen. The lowest effective concentration of recombinant human LH was 10-50 ng/ml, and at 1000 ng/ml 97% of the oocytes underwent GVB. Recombinant human FSH was also effective, albeit slightly less so than recombinant human LH. Follicular oestradiol accumulation was stimulated up to 5-fold, both gonadotrophins showing similar effectiveness. Follicular progesterone and testosterone accumulation was markedly stimulated by recombinant human LH, giving a maximal 20-fold and 15-fold increase respectively. A 5-fold stimulation of progesterone production was found with recombinant human FSH. When submaximal concentrations of recombinant human LH and recombinant human FSH were combined, an additive effect was found on oocyte meiosis and on oestradiol and progesterone accumulation. The results are in agreement with previous studies using pituitary gonadotrophin and show that both recombinant human LH and recombinant human FSH have intrinsic effects on oocyte meiosis and steroidogenesis. It is likely that in the physiological situation in vivo, both gonadotrophins contribute to regulate these events in the peri-ovulatory period.