Courtship declines and ceases while parental care increases in the presence of developing eggs during the nesting cycle of the male three-spined stickleback, Gasterosteus aculeatus. Furthermore, circulating 11-ketotestosterone (11KT) levels are higher during the initial "courtship phase" than during the later "parental phase," similar to that found in other paternal fishes. This study aimed to investigate a possible functional relationship between changes in 11KT levels and changes in reproductive behavior during the nesting cycle. To this end, groups of nonspawned and spawned male sticklebacks were sham-operated, castrated, or castrated and treated with 11-ketoandrostenedione (11KA), and the effects of the treatments on courtship and parental care were studied. Castration removed circulating 11KT, while 11KA replacement prevented the natural decline in 11KT during the parental phase (11KA converts to 11KT extratesticularly), as assessed by radioimmunoassay. Regardless of treatment, parental care remained low and courtship was present in all nonspawned males, even at the end of the experiment. However, courtship did eventually decline in castrated nonspawned males compared to the other two nonspawned groups. In all treatments of spawned males there was a drastic decline in courtship and an increase in parental care. In castrated spawned males, however, the decline in courtship came earlier than in the other two spawned groups. 11KA treatment did not prevent the natural decline in courtship/increase in parental care in spawned males, indicating that the natural decline in 11KT is not responsible for the main portion of the rapid changes in these behaviors over the stickleback's nesting cycle. The limited effects of castration also exclude other gonadal hormones from being responsible for most of these changes.