SUMMARY Human luteinizing hormone (LH) was given by intravenous infusion to four normal male volunteers. The disappearance of immunoreactive LH from serum followed a single exponential decay, the mean half-life being 136 min. The effect on the testis of the infused LH was variable. Despite high serum LH levels being achieved, only one subject showed a clear increase in circulating 17β-hydroxyandrogen (17-OHA) levels, the other subjects showing little change in the immediate post-infusion period. All subjects had low 17-OHA levels in the period 24–36 h after the infusion. No consistent changes in serum follicle-stimulating hormone levels occurred during or after the infusion. This suggests that other factors besides LH are necessary to produce maximal testosterone secretion by the testis, and may also be concerned in controlling the diurnal variation of testosterone.