Previous studies have shown that sexual activity increases the weight of the accessory sex glands significantly, while the organ weights correlate inversely with the assayable androgen receptor concentrations in the prostate of sexually active versus sexually resting rats. In an effort to determine the structural basis of this phenomenon, the ventral prostates of adult rats kept with female rats for 4-6 months (HE-rats) were compared to those kept in groups of 5 males in one cage (HO-rats) for the same period. As an estimate of the secretory function of the gland the concentration of prostatic binding protein (PBP) was determined in prostatic cytosols using a highly specific ELISA. Catecholamines were measured by means of HPLC and subsequent electrochemical detection. Morphological studies included immunocytochemistry of PBP, visualization of adrenergic nerves, stereological analysis of stromal and glandular compartments of the prostate, and electron microscopy of the epithelium. The main findings were as follows: 1) The prostates of HE-rats were 35% heavier than those of the HO-rats. 2) The content in secretion was in the same range in both HE-rats and HO-rats (1.5 and 1.44 mg PBP per 1 mg DNA). 3) Immunocytochemistry and electron microscopy demonstrated a very homogeneous secretion within the glandular lumen of HO-rats with a diminished amount of secretory material within the glandular cells. In HE-rats the glandular lumina were clearly larger in diameter and intraluminal secretion was less homogeneously stained. The height of the epithelium was increased and the individual secretory cells contained several secretory granules.(ABSTRACT TRUNCATED AT 250 WORDS)