The influence of gonadotropin releasing hormone and Decapeptyl on steroidogenic activity was evaluated at concentrations of 0.1 ng/ml, 1 ng/ml and 10 ng/ml culture medium. In the granulosa cell cultures of 9 out of 18 patients addition of Decapeptyl or gonadotrophin releasing hormone to the culture media caused dose-dependent inhibition of progesterone and oestradiol secretion in vitro. The steroidogenic activity in vitro of the granulosa cells of 84 IVF cycles was correlated to the maximal serum E2 levels of the patients at induction of ovulation. The granulosa cells of patients who were stimulated including Decapeptyl produced on average higher E2 levels in vitro as compared to granulosa cells from hMG stimulation cycles. This may be regarded as an indirect clue to an inhibitory effect of Decapeptyl on oestradiol synthesis in vivo.