The immunohistochemical distribution of substance P (SP) in the hypothalamic suprachiasmatic nucleus (SCN) was studied in adult male and female Djungarian hamsters (Phodopus sungorus) held under either long or short photoperiods. Intact animals were killed by perfusion with a fixative at the middle of the light or dark periods, respectively. The tissue was processed by routine immunohistochemical methods. Perikarya exhibiting SP-like immunoreactivity (LI) were found in the SCN of animals of all groups. These cell bodies predominantly were restricted to a distinct portion of the nucleus extending less than 150 microns rostrocaudally and were often concentrated in its lateral aspect. SP-LI fibers were rarely observed in the SCN, however, other hypothalamic parts, e.g. anterior and paraventricular regions, exhibit strong SP-LI innervation patterns. Sex-related differences were not observed. Long-term exposure to short days decreased the number of neurons exhibiting SP-LI by approximately 60% when compared to long-day animals at both day- and nighttime. At night, SP-LI neurons were augmented in number by 34% (long-day group) and 56% (short-day group). Further, the numbers of SP-LI perikarya in the SCN of aged hamsters at day- and nighttime were augmented 3- to 4-fold when compared to adult animals. These results suggest that substance P in the SCN is involved in the regulation of circadian and seasonal mechanisms in this highly photoperiodic rodent species.