In an earlier study, normal adult men were shown to have increased plasma testosterone (T) levels over a several-hour period after haloperidol-induced increases in plasma PRL levels. The present study was designed both to replicate our first study and to examine the potential synergism of PRL and LH in influencing T levels on a short term basis in normal men. Eight volunteers received on 4 separate days an in injection of saline or 0.5 mg haloperidol at 1000 h and an iv injection of saline or 88 IU human LH (hLH) at 1100 h in a double blind randomized block design arranged to augment plasma levels of PRL, LH, and PRL and LH together on the different test days as well as to afford a saline control day. Only five of the eight subjects had prompt PRL responses to haloperidol equivalent to those of our earlier study. As the purpose of this study was to examine the effect of increased PRL on plasma T levels, these five subjects were used for the determination of changes in plasma T. After haloperidol administration, their PRL levels rose an average of 19 ng/ml, to the high-normal range, and after the hLH infusions, their LH levels rose an average of 71 ng/ml. On the saline control day, mean T levels showed the normal diurnal decline. After 0.5 mg haloperidol, T levels were maintained for several hours, and after 88 IU hLH, T levels were increased for several hours. Increased PRL levels concomitant with hLH administration did not produce a T response greater than that caused by hLH alone. The results of this study replicate the effect of drug-induced PRL augmentation on plasma T levels found in our earlier study, but they fail to demonstrate a synergistic effect of acutely increased PRL on LH-stimulated T secretion. PRL thus seems to be another pituitary hormone capable of increasing plasma T in adult men, but it clearly is a weaker stimulus than LH.