Transforming growth factor 1 (TGF1) plays an important role in the growth of the prostate, and has been reported to stimulate or inhibit proliferation of prostatic epithelia. We show here that Tgf1, Tgf2 and Tgf3 mRNA expression correlated with developmental growth of the prostate. Recombinant TGF1 inhibited the growth of the prostate when added to cultures of ventral prostate (VP) organs grown in vitro. Interestingly, TGF1 had contrasting effects upon cellular proliferation; it stimulated proliferation at the periphery of the organs (distal to urethra) but inhibited proliferation in the center of the organs (proximal to urethra). We speculate that differential effects upon proliferation may be determined by the level of cellular differentiation, since cells at the periphery are undifferentiated while those in the center are more highly differentiated. TGF1 also stimulated branching morphogenesis at growing ductal tips at the perimeter of the VP. To investigate potential mechanisms of TGF1 action, we examined the three dimensional distribution of smooth muscle in prostatic organs following treatment with TGF1. TGF1 showed a significant effect upon the distribution of smooth muscle within VPs which may mediate part of its' effect upon proliferation. Finally, we addressed how testosterone and TGF1 might affect gene expression in our developmental system. Testosterone repressed the expression of Tgf2 mRNA in the prostate, while TGF1 showed a modest repression of Fgf10 mRNA. It appeared that effects of these factors were more pronounced in a model of prostatic mesenchyme devoid of epithelia compared with prostatic organs (containing epithelia).