In order to investigate the endocrine requirements for induction of follicle maturation, ovulation and luteal function we have measured plasma LH and FSH concentrations in 109 cycles induced by treatment with LHRH in 25 women with amenorrhoea of diverse aetiology. The major clinical and endocrine subgroupings were polycystic ovarian disease (PCO), Kallmann's syndrome, weight related amenorrhoea and hyperprolactinaemia. By analysis of variance of the LH and FSH concentrations obtained three times per week in the follicular phase in 77 ovulatory cycles (including 24 conception cycles), we found that in women with PCO the mean LH concentration was 19.5 IU/l. In hyperprolactinaemia and secondary amenorrhoea of non-specific aetiology the mean LH concentration was 11.6 IU/l and in patients with hypogonadotrophic hypogonadism and weight-related amenorrhoea it was 7.2IU/l. These mean LH concentrations were significantly different from each other (P less than 0.001). We speculate that the high LH concentrations in the follicular phase may impair the final stages of oocyte maturation and so contribute to the infertility of these patients. FSH concentrations in the follicular phase were lower in patients with hypogonadotrophic hypogonadism and weight-related amenorrhoea than in other groups. Although the differences were statistically significant (P less than 0.001) the difference was small (mean FSH in hypogonadotrophic hypogonadism and weight-related amenorrhoea 4.3 IU/l vs 6.3 IU/l in the others) and of uncertain biological significance. During the luteal phase, no differences between any of the groups in LH and FSH concentrations were found.