Nitric oxide plays a critical role in the immune response, and our studies have shown that heroin induces a reduction in the expression of iNOS, the enzyme responsible for nitric oxide production. The present study evaluated the effect of heroin self-administration on iNOS expression using a three-group design. Group one (self-administration) was trained to press a lever for i.v. administration of heroin. Group two (yoked heroin) received a simultaneous equivalent infusion of heroin determined by the responses of a 'partner' animal in the first group. A third group (yoked saline) also was yoked to the first group, but received i.v. injections of saline. Immediately following the last session, all rats received an injection of lipopolysaccharide (LPS) to induce iNOS expression. About 6 h after the injection of LPS, iNOS mRNA and protein expression were determined in spleen, lung, and liver. Additionally, the accumulation of plasma nitrite/nitrate, the more stable end products of nitric oxide degradation were measured. Although there was not a consistent difference between the self-administering and yoked-heroin animals, the results show that rats will self-administer a sufficient amount of heroin to induce a pronounced, widespread reduction in the expression of iNOS.