In the present study, the ajmaline test was applied to T. cruzi-infected rats and evaluated for the ability to reveal ECG disturbances. The test consists of intravenous injection of ajmaline (2 mg/kg body-weight) under ether anaesthesia, and continuous ECG monitoring (right precordial lead: V1 or V2, 100 mm/sec paper speed). The animals used for the test (n = 30) had been experimentally infected with the Colombia strain of T. cruzi (1,000 parasites/g body-weight, newly weaned rats) one year before the experiments. Control non-infected rats (n = 21) were similarly maintained and treated. The ECGs were analysed under baseline conditions, i.e., before ajmaline, and 15, 30, 60 and 180 seconds after completion of ajmaline injection through the dorsal vein of the penis. The following parameters were studied: heart rate, PR interval, QRS and Qat duration. Morphological changes of the QRS complex and ST-T segment were also recorded. Under baseline conditions, the two groups had comparable values for all parameters, except for the PR interval, which was significantly increased in infected rats. No changes in wave rhythm or morphology were detected under baseline conditions. The PR and QaT intervals and QRS duration were significantly longer in both groups after ajmaline injection, at all periods studied. When the two groups were compared for relative variations, consistently and significantly higher alterations (p less than 0.05) were observed in the T. cruzi-infected group, except for the QaT interval at 180 seconds. However, the severe rhythm and ventricular conduction disturbances detected in 30% of the infected rats represented clear-cut discriminative alterations.(ABSTRACT TRUNCATED AT 250 WORDS)