Serine protease inhibitor Kazal-type (SPINK3)/P12/PSTI-II is a small secretory protein from mouse seminal vesicle which contains a KAZAL domain and shows calcium (Ca(2+))-transport inhibitory (caltrin) activity. This molecule was obtained as a recombinant protein and its effect on capacitated sperm cells was examined. SPINK3 inhibited trypsin activity in vitro while the fusion protein GST-SPINK3 had no effect on this enzyme activity. The inactive GST-SPINK3 significantly reduced the percentage of spermatozoa positively stained for nitric oxide (NO) with the specific probe DAF-FM DA and NO concentration measured by Griess method in capacitated mouse sperm; the same effect was observed when sperm were capacitated under low Ca(2+) concentration, using either intracellular (BAPTA-AM) or extracellular Ca(2+) (EDTA) chelators. The percentage of sperm showing spontaneous and progesterone-induced acrosomal reaction was significantly lower in the presence of GST-SPINK3 compared to untreated capacitated spermatozoa. Interestingly, this decrease was overcome by the exogenous addition of the NO donors, sodium nitroprusside (SNP), and S-nitrosoglutathione (GSNO). Phosphorylation of sperm proteins in tyrosine residues was partially affected by GST-SPINK3, however, only GSNO was able to reverse this effect. Sperm progressive motility was not significantly diminished by GST-SPINK3 or BAPTA-AM but enhanced by the addition of SNP. This is the first report that demonstrates that SPINK3 modulates sperm physiology through a downstream reduction of endogenous NO concentration and independently of SPINK3 trypsin inhibitory activity.