In the present study we investigated the effects of various tachykinins on the secretory activity of rat Sertoli cells in vitro. Sertoli cells were isolated from testes of immature Sprague Dawley rats, cultured for 4 days and thereafter incubated with three concentrations (0.1 pM, 1 pM or 100 pM) of substance P (SP), neurokinin A (NKA), neuropeptide K (NPK) or neuropeptide gamma (NPG) for 24 h. Levels of transferrin and lactic acid were determined in the culture media and expressed per micrograms of cellular DNA. Among all the peptides studied, NPG exhibited the greatest stimulatory effect on the release of transferrin and lactate, with NKA and NPK being less potent and SP being the least potent. Also, the effects of tachykinins on the aromatase activity of cultured Sertoli cells, as reflected by their ability to metabolize testosterone to estradiol (E2), were studied. No stimulatory effect was observed at lower concentrations (1 pM), while at 100 pM both NPG and NKA increased estradiol levels in the medium. SP and NPK had no significant effect on estradiol levels in the medium. This study reveals that tachykinins are able to influence the secretory activity of Sertoli cells, and that some of these peptides can also enhance the aromatase activity. Thus there is a possibility that tachykinins may have a physiological role as modulators of the function of Sertoli cells.