Estradiol (E2) levels in systemic plasma of both female and male ring doves (Streptopelia risoria) were determined with a radioimmuoassay during periods of isolation, courtship, nest building and the incubation of eggs. In isolated females the plasma levels of E2 are low (averaging less than 40 pg⁄ml). Upon introduction to a male E2 levels in females rise significantly during courtship (mean 85 pg⁄ml ± 11 SEM) are high during nest building (mean 67 pg⁄ml ± 9 SEM) and are not detectable during incubation (less than 25 pg⁄ml). In contrast E2 is not detectable in the plasma of males at any of these stages of the breeding cycle. Therefore, in females, endogenous plasma estrogen is available to support the increase in nest soliciting display during courtship and to play a role in the transition to nest building behavior. However, in males it seems highly unlikely that endogenous estrogen could be important in the support of nest soliciting or nest building as has been proposed by others on the basis of systemic injections and hypothalamic implants of estrogens. In castrated males injected with testosterone propionate (TP, 200 μg⁄day), estradiol benzoate (EB, 200 μg⁄ day), or sesame oil (SO) for 23 days, only EBinjected birds had detectable plasma E2. Although both TP- and EB-treated castrated males nest solicit and nest build, extragonadal conversion of testosterone to plasma E2 appears not to be responsible for the ability of androgens to produce nest soliciting and nest building. Since in males androgens are sufficient to support courtship and nest building displays and endogenous circulating E2, if any, is so much lower than in females, it is concluded that there is no need to presume a role for circulating estrogen in courtship and nest building in male ring doves. (Endocrinology94: 1126, 1974)