Wilson's phalarope (Phalaropus tricolor) is characterized by intense female intrasexual competition and exclusive male parental care. Females occasionally are polyandrous and no territories are defended. Prolactin (Prl) and testosterone (T) were analyzed from plasma samples obtained from individuals of different reproductive stages. Males tended to have higher plasma Prl levels than females throughout the breeding season. Prolactin levels of males declined posthatch, reaching baseline levels when chicks were approximately 9 days old-the time when chicks approach thermal independence and brooding is minimal. Testosterone levels dropped at the onset of incubation and remained low through the brooding period. Eggs were manipulated such that some males incubated clutches for shortened periods, others for extended periods. Regardless of the length of incubation, Prl levels were determined primarily by environmental events, i.e., incubation, hatching, and brooding.