We investigated the effects of induced increase in prolactin levels on spermatogenesis in 20 infertile men with hypoprolactinemia using exogenous human prolactin (hPRL) and metoclopramide. The subjects were selected from a population of 175 infertile men in whom the prevalence of hypoprolactinemia was 33.14%. Mean basal plasma prolactin was 2.79 +/- 0.62 ng.ml-1 in the infertile men and 9.57 +/- 2.14 ng.ml-1 in the normal control subjects. At the sixteenth week, mean plasma prolactin was 9.41 +/- 1.3 ng.ml-1 in subjects treated with exogenous hPRL and 5.2 +/- 0.7 ng.ml-1 in subjects treated with metoclopramide. Mean basal sperm concentration was approximately 8.8 million per milliliter in the infertile men and 41.5 million per milliliter in the normal control subjects. Mean sperm concentration was approximately 37 million per milliliter in subjects treated with exogenous hPRL, whereas the peak mean value was 23 million per milliliter in subjects treated with metoclopramide for 16 weeks. At basal conditions, the mean percentages of abnormal sperm were 66.75% +/- 14.93% and 21.36% +/- 4.78% in infertile and normal subjects, respectively. In subjects treated with exogenous hPRL and metoclopramide, the mean percentage of abnormal sperm were 24.7% and 31%, respectively, at week 16. Mean plasma prolactin, mean sperm concentration and the mean percentage of abnormal sperm were 3.3 +/- 1.4 ng.ml-1, 7 million per milliliter, and 60.5, respectively, in the infertile subjects after drug withdrawal at week 14. In normal control subjects, there was no significant difference (p = 0.01) in the plecebo effect. We therefore conclude that the low prolactin levels in this group of infertile men may be one of the primary causes of their infertility.