Recent immunocytochemical studies have suggested that vasopressin (VP) neurons in the bed nucleus of the stria terminalis (BNST) of the rat are gonadal steroid sensitive. In this paper we have used in situ hybridization and quantitative autoradiography to determine whether testosterone (T) and/or its metabolites modulate the biosynthetic capacity of VP neurons in the BNST of adult male rats. In Exp 1 the number of labeled cells and the average number of grains per cell were compared in sections sampled through the BNST of intact, castrated, and castrated male rats treated with physiological levels of T (1.6 +/- 0.1 ng/ml plasma). Castration dramatically reduced the number of labeled cells (P less than 0.01) and the intensity of labeling (P less than 0.05) of cells in the BNST. T, treatment of castrated animals reversed the effect of castration on both cell number and grains per cell. In Exp 2 treatment of castrated rats with supraphysiological levels of T (7.6 +/- 0.7 ng/ml plasma) increased the number of labeled BNST cells (P less than 0.05) and the intensity of labeling (P less than 0.05) over those in castrates treated with physiological levels of T or intact rats. These results indicate that T and/or its metabolites modulate expression of the VP gene by neurons in the BNST of adult male rats.