Siberian hamsters (phodopus sungorus sungorus) show naturally occurring seasonal cycles of food intake that are triggered by changes in the photoperiod. In long "summer-like" days (LD) food intake is at its peak, whereas in short "winter-like" days (SD) food intake reaches a nadir. Although the mechanisms underlying these changes in food intake are unknown, results from previous studies suggest that the ability to stimulate or inhibit food intake in Siberian hamsters complements the naturally occurring food intake cycle. Thus, inhibitors of food intake are more effective in SDs, whereas stimulators of food intake are more effective in LDs. A stimulator of food intake in a wide variety of species is neuropeptide Y (NPY). Therefore, we explored the ability of NPY to stimulate food intake in Siberian hamsters. In addition, we tested whether the efficacy of NPY to stimulate food intake was photoperiod dependent. In Experiment 1, LD-housed adult male hamsters were given a series of NPY doses (0.078-10.0 micrograms) intracerebroventricularly (ICV) into the third ventricle and food intake was measured 30 min, 1, 2, and 4 h postinjection. NPY was a potent stimulator of food intake with the 7.5 micrograms dose of NPY producing the greatest increase at 30 min. In Experiment 2, adult male hamsters were housed in LDs or SDs and were given various doses of NPY ranging from 0.039-7.5 micrograms. NPY given ICV stimulated food intake to the same degree in LDs as in SDs with the greatest increases in food intake occurring in the hamsters receiving the 2.5 and 5.0 micrograms dose of NPY. In addition, Siberian hamsters were very sensitive to NPY with the lowest effective dose (0.0585 microgram) that stimulated food intake being six times smaller than in other rodents tested. Collectively, these results showed that Siberian hamsters were more sensitive to the stimulatory effect of NPY on food intake than any other species, but that the ability of NPY to stimulate feeding was not photoperiod dependent.