Behavioral and endocrine changes associated with reproductive events were studied in free-living male house sparrows (Passer domesticus). Circulating levels of luteinizing hormone (LH), testosterone (T), and dihydrotestosterone (DHT) were maximal during egg-laying and declined during incubation and the first 2/3 of the nestling stage. As the young approached fledging, levels of LH and T rose to levels similar to those of the first egg-laying stage. This pattern was repeated three to five times during the prolonged breeding season of this species. Multivariate statistical analysis demonstrated that elevated levels of LH and androgens during egg-laying stages were associated with high rates of intrusion at nests by conspecifics, especially other adult males, and elevated levels of agonistic activity, nest defense, and mate-guarding behavior by breeding males. Feeding rates of males declined significantly as plasma levels of T began to rise. Concentrations of corticosterone (B) were high during each egg-laying and nestling stage and were correlated with high or rising levels of reproductive hormones. This suggests that reproductive activity, while energetically demanding, was not overly stressful to these birds. Body mass and fat depots were lowest during the final brood of the season. We suggest that the temporal pattern of circulating levels of T in male house sparrows is an adaptation which compromises between two conflicting selective pressures: a high level of male-male competition for limiting nesting sites in a semicolonial setting, and a strong demand for parental care associated with large broods of altricial young.