In some tissues 17beta-estradiol (E2) is known to increase endothelial NOS expression. In the present study we examined the effects of E2 on estrogen receptors (ERalpha and beta) and inducible nitric oxide synthase (iNOS) expression and analyzed the mechanisms in rat peritoneal macrophages. Reverse-transcription polymerase chain (RT-PCR) and transient transfection experiments using a reporter plasmid that contained a luciferase gene under the transcriptional control of an estrogen-responsive elements revealed that peritoneal macrophages are responsive to E2 and express both ERalpha and ERbeta mRNAs. Incubation with E2 leads to an increased ERbeta mRNA expression. When rat peritoneal macrophages were incubated with physiological concentrations of E2, E2 induced a dose-dependent increase in NO production. E2 significantly affected secretion at concentration levels of more than 10(-11)M, and its maximum effect was at a concentration of 10(-8)M. RT-PCR reactions showed that increases in NO secretion were due to an increase in iNOS mRNA. Coincubation with ICI 182.780, an estrogen-receptor antagonist, inhibited the influence of E2 on NO production and iNOS expression. Thus E2 stimulated iNOS expression by a classic receptor-mediated pathway. We hereby prove that E2 increases the iNOS expression in macrophages and this effect appears to be the consequence of ER activation.