Our previous studies suggest that increased serum PRL, secondary to haloperidol-induced dopamine blockade, augments serum testosterone (T) levels in normal men. To rule out a direct effect of haloperidol on the testis, serum samples from a methyl-TRH study in normal men, in whom serum PRL levels were increased by a stimulus other than dopamine blockade, were analyzed for T. Fourteen subjects received both a low dose (6.25-12.5 micrograms) and a high dose (100-500 micrograms) of methyl-TRH on separate days; blood sampling was done for 15 min before and for 4 h after drug infusion. Compared to a saline control group of 14 normal men, who showed a diurnal decline of serum T levels, the methyl-TRH treated subjects had statistically significant increases in serum T after both low and high doses. These data provide further support for the concept that PRL is a pituitary hormone capable of augmenting serum T levels in normal adult men.