Experimental evidences shows the presence of ultradian rhythms with period range from 2,5 and 20 minutes in RTs in sustained attention, with large inter-subjects variability. Differently from circadian rhythm, the ultradian rhythms does not apparently match with any environmental periodicity, and it is not known any factors that modulate it. The study of the external factors influence on ultradian parameters could clarify about the endogenous processes involvement in the generation and modulation of ultradian rhythms. In the 3 study proposed here, it was investigated the presence of ultradian rhythms in sustained attention in visual modality, trying to enrich their characterization by the study of the effect of taskload. Results have demonstrated the existence of ultradian stable rhythms in sustained attention, and it was also observed the presence of a differential modulation of task-load on such rhythms.