California mice (Peromyscus californicus) breed primarily during the winter rainy season and generally terminate breeding during the dry summer months. This pattern of reproduction could be regulated by day length, availability of green vegetation, or water availability. The effects of photoperiod and green vegetation on reproduction were examined in Experiment 1 by housing adult male P. californicus either in long (LD 14:10) or short (LD 8:16) photoperiods for 10 weeks with ad lib food and water availability. A subset of animals in each photoperiod treatment group also received supplements of fresh spinach thrice weekly. The effects of water availability were examined in Experiment 2 by housing adult males in long day length conditions for 10 weeks with ad lib or restricted (50% of ad lib) water availability. Neither photoperiod nor availability of green plant food significantly affected reproductive function in male California mice, although animals in long days with green food supplements displayed elevation of some reproductive organ masses. Short days did not suppress plasma LH or prolactin levels. Male P. californicus provide extensive care of the young during the short days of winter. The absence of photoperiod-induced changes in prolactin levels is consistent with the observation that elevated plasma prolactin titers are associated with male parental care in this species. In contrast, water restriction (simulated summer drought) reduced reproductive organ masses, as well as plasma levels of prolactin, and may act as an environmental cue to terminate breeding. Thus, water availability may regulate breeding in this species independently of photoperiod and food availability.