Testicular biopsy specimens from 11 infertile men were incubated in vitro with [3H]progesterone before and during long term gonadotrophic treatment. The main metabolites along the Δ‐4 metabolic pathway and 20α‐dihydro‐progesterone were determined and the ratio between 20α‐dihydro‐progesterone and 17α‐hydroxyprogesterone formed in vitro was calculated. In 5 patients with originally high ratios (indicating gonadotrophic understimulation), steroid metabolism changed significantly towards a more stimulated pattern. Three of these patients also showed a significant increase in sperm output and 2 of them fathered children. In 6 patients with an originally low ratio (indicating adequate gonadotrophic stimulation) no change in steroid metabolism in vitro or spermatogenesis was seen during therapy. Thus, this ratio between formed 20α‐dihydro‐progesterone and 17α‐hydroxyprogesterone in vitro seems to be of value in predicting whether gonadotrophic treatment will