The suprachiasmatic nucleus (SCN) is thought to be the main neuronal oscillator underlying circadian rhythmicity of different biological phenomena such as sleep-wakefulness and body temperature. Although numerous studies in old rats showed that circadian organization is clearly disturbed in senescence, no decrease in total SCN cell number has been observed. However, in an earlier study we found a significant decrease of approximately 30% in the number of immunocytochemically-stained vasopressin (VP) neurons in the SCN of the old rat. The aim of the present study was to examine whether another group of SCN neurons, i.e., the vasoactive intestinal polypeptide (VIP) cells, shows age-related changes parallel with disturbances found in sleep/wake parameters. Immunocytochemical staining with antiVIP followed by morphometric analysis revealed a 36% decrease in the number of immunoreactive VIP neurons in the SCN of old rats as compared to young ones. The average size of the remaining VIP cells increased in aged rats. The rapid-eye-movement (REM)-sleep time was negatively correlated with the immunoreactive VIP cell number in the old animals. VP and VIP alterations in the SCN may constitute an anatomical substrate for the circadian disturbances observed in senescence.