Enriched populations of germ cells prepared from adult rats were found to influence 20-day-old rat Sertoli cell secretory activity by stimulating androgen-binding protein (ABP) and inhibiting oestradiol-17 beta production in the presence of follicle-stimulating hormone (FSH) as well as of dibutyryl cyclic AMP (dbcAMP). Among the different populations tested in coculture, pachytene spermatocytes were the most effective at stimulating ABP and inhibiting oestradiol production, whereas early spermatids had relatively less effects. Cytoplasts from elongated spermatids only slightly stimulated ABP secretion. The influence of germ cells upon Sertoli cells may be mediated via paracrine component(s) detected in nonconcentrated conditioned culture media. The stimulatory (ABP) and inhibitory (oestradiol) effects of pachytene spermatocyte and early spermatid-spent media were reversible (change of media), dose related, specific (no effect of cytoplast, peritubular cell, rat liver epithelial cell or 3T3 cell-conditioned media) and strictly proportional to the cell viability estimated at the end of the incubation periods. Furthermore, the nature of the germ cell factor(s) influencing Sertoli cell secretory function is likely to be proteinaceous since both germ cell-spent media effects were trypsin and heat (100 degrees C; 3 min) sensitive and retained by molecular weight (MW) greater than 10,000 cut-off dialysis membranes. It is hypothesized that germ cells, in particular pachytene spermatocytes and early spermatids, may influence Sertoli cell function during sexual development in the rat.