The sexually dimorphic vasopressin system of the bed nucleus of the stria terminalis (BNST) is the most sensitive neurotransmitter system regulated by sex steroids in rats and mice. In addition to vasopressin, the BNST neurons also express a second neuropeptide, galanin, whose expression also appears to be regulated by testosterone in laboratory rodents. Seasonal fluctuations of sex steroids in photoperiodic rodents feed back on the brain to regulate the expression of sex steroid sensitive genes. The seasonal rhythm of circulating sex steroids is generated by photoperiod-controlled melatonin secretion, resulting in a seasonal stimulation and involution of the gonads. We have studied the seasonal expression of vasopressin and galanin in BNST neurons and their target areas in the Djungarian hamster (Phodopus sungorus). Furthermore, we analyzed the effect of testosterone on vasopressin and galanin by testosterone supplementation in animals where reproduction was inhibited by exposure to a short photoperiod. Exposure to short photoperiod induced a major reduction in the expression of vasopressin in BNST neurons, as well as in their target areas, the lateral septum (LS) and the lateral habenula (LHb). Galanin expression in the BNST and its target areas was also strongly reduced, although this reduction did not result in an almost complete disappearance of the neuropeptide as observed for vasopressin. Testosterone was able to reverse this reduction for both vasopressin and galanin. However, while the mRNA expression in BNST neurons recovered within 2-4 days, recovery of the neuropeptide immunoreactivity in the target areas, LS and LHb, required more than 3 weeks. The photoperiod-driven testosterone rhythm thus appears to be a major regulator of extra-hypothalamic vasopressin and galanin in the Djungarian hamster. The long delay between mRNA recovery in the cell body and the neuropeptide recovery in the target areas may be due to progressive filling up of the axon terminals. Alternatively, this delay might be indicative of a seasonal structural plasticity.