To characterize the kinetics of myocardial cytokine and inducible nitric oxide synthase (iNOS) expression in acute Chagasic cardiomyopathy, we studied a rat model of acute Trypanosoma cruzi infection. Rats were euthanized 36 hours and 5, 10, and 15 days after infection, and hearts were collected for histology, mRNA, and protein analyses. Histological analysis of myocardium showed a progressive increase in the number of amastigotes and mononuclear inflammatory cells. Organisms were first detected 5 days after intraperitoneal inoculation as isolated nests and became numerous by day 15. Northern blot analysis of total RNA revealed no signal for interleukin (IL)-1beta or tumor necrosis factor (TNF)-alpha and a weak signal for IL-6 in control hearts. High levels of expression for the three genes were detected in the infected animals at 36 hours after infection. Although IL-1beta and IL-6 levels increased steadily up to 10 days, TNF-alpha levels were the highest at 5 days, remained high at 10 days, and declined thereafter. Western blot analysis showed similar results to that of mRNA expression. No signal was detected for iNOS in the controls, but both its mRNA and protein were found in the infected animals, with levels being highest at 15 days after infection. Immunohistochemistry revealed no iNOS immunoreactivity in uninfected animals, but intense iNOS staining was detected in blood vessels of infected animals, which decreased progressively with period of infection. Positive staining for iNOS in cardiomyocytes was first detected at 36 hours after infection (at a time when there was no histological inflammatory reaction), which steadily increased, being the highest at 15 days after infection. These results indicate that, in addition to mechanical damage by T. cruzi, substantial pro-inflammatory cytokine production within the myocardium is likely to participate in the pathophysiology of acute Chagasic cardiomyopathy.