To examine the fundamental rhythms of sympathetic discharges, the periodicity of synchronized renal nerve activity (RNA) was measured in anesthetized cats. After filtering the RNA between 50-3000 Hz, rectification and integration the periodicity was calculated by measuring the peak to peak intervals. Two major periodicities were detected, a Tc rhythm between 6 and 25 c/s with a 43% probability and a Tb rhythm between 2 and 6 c/s with a 57% probability. The mean periods of Tc and Tb were 95 +/- 2 ms and 297 +/- 4 ms (+/- SE) respectively. Another techniques which has previously been used to assess the periodicity was also applied, in which the original neurogram was filtered between 0.08-3000 Hz and measured by power spectral analysis. However, this technique only identified the 2-6 c/s frequency and the faster Tc rhythm was undetected. Baroreceptor stimulation induced by noradrenaline decreased the probability of Tc and Tb modal components. Baroreceptor denervation led to an increase in the probability of the Tc rhythm and reduction in the Tb rhythm. These results support our model that the Tc rhythm reflects a fundamental periodicity of central origin and that the Tb rhythm reflects a periodicity of cardiac-related RNA, which is produced by reflex inhibition of the fundamental rhythm by periodic baroreceptor input.