The aims of this study were to measure the activity of the autonomic nervous system using heart rate variability (HRV) during learning tasks and to clarify the relationship between learning to overcome a difficult situation and the autonomic nervous system in monkeys. Two young male monkeys (Macaca mulatta) were given simple discrimination learning tasks (DL) and delayed matching to samples tasks (DMTS); Holter-type electrocardiography was done, and HRV was measured. We defined the frequency bands of HRV in rhesus macaques; the low frequency (LF) was 0.01-0.15 Hz, and the high frequency (HF) was 0.15-0.50 Hz. Based on these frequency bands, the LF/HF ratios during learning tasks were analyzed, and a significant increase in the ratio was found during the tasks. The variances in the HF differed between the DL and DMTS tasks; during DMTS tasks, HF variances had a tendency to increase. Our results indicate that increased sympathetic activity accompanied learning and suggest that the parasympathetic nervous system plays a key role during learning, particularly when difficult tasks are being learned.