Eider ducks showed clear tidal and seasonal cycles of display when involved in pair-formation behavior. Plasma androgen (testosterone and dihydrotestosterone) concentrations did not follow similar tidal cycles, but there was a 4-fold increase of androgen in spring when rates of display increased 2-fold. There was no difference in androgen levels in blood samples taken from paired birds before and after coitus. Androgens therefore appear to be essential for the expression of sexual behavior, but there is no apparent quantitative correlation between overt sexual activity and androgen concentrations.