The morphology and functional activity of rat seminal vesicles from normal animals were compared to those administered 3 weeks the synthetic ergot alkaloid lisuride hydrogenmaleate (LHM) with or without simultaneous prolactin substitution and to those from rats castrated 3 days previously. The secretory cells in seminal vesicle epithelium of LHM-treated animals showed regressive changes, irrespective to simultaneous prolactin treatment. Castration resulted in a sharp drop of serum testosterone and consequently in a significant decrease in protein biosynthesis of tissue slices. LHM treatment depressed serum prolactin levels, but testosterone levels and protein biosynthesis were in the normal ranges. In LHM-treated, prolactin-substituted animals both the prolactin and testosterone levels were low, but amino acid incorporation of tissue slices was unaltered. The results indicate that the expected changes of prolactin depression on seminal vesicle functions are obscured by the toxic effects of LHM. LHM treatment is therefore inappropriate for the study of prolactin depletion on the functions of the rat seminal vesicle.