Natural variation in neuroendocrine traits is poorly understood, despite the importance of variation in brain function and evolution. Most rodents in the temperate zones inhibit reproduction and other non-essential functions in short winter photoperiods, but some have little or no reproductive response. We tested whether genetic variability in reproductive seasonality is related to individual differences in the neuronal function of the GnRH network, as assessed by the number and location of mature GnRH-secreting neurons under inhibitory and excitatory photoperiods. The experiments used lines of Peromyscus leucopus previously developed by selection from a wild population. One line contained individuals reproductively inhibited (RI) by short photoperiod, and the other line individuals nonresponsive (NR) to short photoperiod. Expression of mature GnRH immunoreactivity in the brain was detected using SMI-41 antibody in the singlelabeled avidin-biotin-peroxidase-complex method. NR had 50% more immunoreactive GnRH neurons than RI mice in both short and long day photoperiods. The greatest differences were in the anterior hypothalamus and preoptic areas. In contrast, we detected no significant within-lines differences in the number or location of immunoreactive GnRH neurons between photoperiod treatments. Our data indicate that high levels of genetic variation in a single wild population for a specific neuronal trait are related to phenotypic variation in a life history trait--winter reproduction. Variation in GnRH neuronal activity may underlie some of the natural reproductive and life history variation observed in wild populations of Peromyscus leucopus. Similar genetic variation in neuronal traits may be present in humans and other species.