There are recognized interactions between prostatic stromal and epithelial cells. These interactions may be influenced by the composition of the extracellular matrix, which is composed of proteins such as collagen, laminin, fibronectin, and proteoglycans (PGs) such as chondroitin sulphate proteoglycan (CSPG). In our continuing studies on prostate biology, we examined the three lobes of the normal adult rat prostate, i.e., ventral, dorsal, and lateral, for CSPG by indirect immunofluorescence, using an immunospecific monoclonal antibody (CS-56) for the chondroitin sulphate (CS) moiety of the PG. Staining of the prostate sections with CS-56 antibody followed by labelling with IgG fluorescein isothiocyanate conjugate indicated strong fluorescent signals associated with the ventral lobe basement membrane. The signal was stronger and more continuous in the distal acini than in the proximal acini. The staining of the dorsal and lateral lobes was less intense than that of the ventral lobe. Following castration of the rats, the basement membrane staining became discontinuous. Androgen replacement by administration of testosterone propionate (TP) reversed the effects of castration. Quantification of the total CS content showed decreases of about 60% in the ventral and lateral lobes after castration. TP administration for 14 days increased the total CS content several fold above the values for castrated rats in all lobes. The results demonstrated that CS content was significantly higher for TP-treated animals, suggesting that the expression of prostate CSPG is regulated by androgens. This approach should be useful in the study of the extracellular matrix in prostate biology.