Paramecium tetraurelia exhibit a brief spontaneous backward swimming behavior that is independent of external stimuli. This spontaneous avoiding response (SAR) occurs at regular intervals which increase and decrease in frequency over time. Oscillation of the SAR frequency repeats every 50 minutes and is temperature compensated. Ciliary reversal is triggered by cell depolarization which activates a ciliary voltage-gated calcium channel and is associated with increased intracellular cGMP. Addition of LiCl perturbs the ultradian rhythm of the SAR frequency and myo-inositol restores this rhythm. Thus guanylate cyclase and the inositol signaling pathway may be involved in generation of the ultradian rhythm. Furthermore, addition of theophylline, was found to shorten the oscillation period (tau) and reduce the frequency of avoiding responses overall (mean value). Addition of 8-bromo cGMP was found to also shorten the tau but had no significant effect on the mean value. RNAi silencing of one of the guanylate cyclase genes resulted in elimination of an organized ultradian rhythm and significant reduction in the mean value. These results indicate that guanylate cyclase has a significant impact on the generation of the P. tetraurelia ultradian rhythm.