We investigated the effect of nitric oxide on the catalytic activity of 5'-nucleotidase associated with insoluble membrane domains (rafts) of pig stomach smooth muscle. The low concentration (0.1-10.0 microM) of nitric oxide donor sodium nitroprusside led to essential increase of catalytic activity of 5'-nucleotidase. Maximal increase was observed at concentration of sodium nitroprusside of 1 microM. The enzyme's catalytic activity decreased to about control value at higher concentration of this substance. The catalytic activity of 5'-nucleotidase was also increased at presence of NaNO2, but only at high concentration (10 mM). The specific thiol-alkylating agent N-ethylmaleinimide (1-100 microM) led to essential decrease of enzyme catalytic activity. Our data shows that nitric oxide changes the AMP-ase activity of 5'-nucleotidase, that is thought to be due to direct effect of this substance on protein. We suppose, that such effect of nitric oxide could be physiologicaly important in functioning of smooth muscle.