Plasma progesterone (PROG), testosterone (TEST), oestrone (E1) and 17 beta-oestradiole (E2) concentrations were determined in collared doves living under natural conditions in young as well as sexually inactive animals further in different phases of the reproduction cycle; measurements were made by radioimmunoassay following Sephadex LH 20 chromatography. Sexual steroid plasma concentration in young and sexually inactive male and female birds were essentially the same characterized by relatively high PROG (3--400 pg/ml), and lower and similar TEST and E2 levels (2--300 pg/ml). In males, high TEST (2092 +/- 1074.9 pg/ml) and low E2 (89.4 +/- 57.3 pg/ml) levels were characteristic during the courting phase. In hatching males, although the weight of the testicles were not altered. TEST level markedly decreased 498 +/- 132.9 pg/ml) while E2 level significantly increased (469 +/- 90.1 pg/ml). "Lactating" males were characterized by similar sexual steroid plasma concentration levels, however, the weight of the testicles was greatly reduced. In female doves high PROG (1057 +/- 430.7 pg/ml) and low E2 (96.5 +/- 79.9 pg/ml) levels were found during the growing period of the follicles. In birds with ovaries having large follicles PROG level were depressed (487.7 +/- 255.6 pg/ml), while that of E2 increased (337 +/- 140.2 pg/ml). In periovulating birds high PROG (2972.5 +/- 1134.9 pg/ml) and low E2 (73 +/- 82.4 pg/ml) levels were detected. Both PROG (478.5 +/- 98.4 pg/ml) and E2 (101 +/- 45.5 pg/ml) levels were low in hatching and lactating females. Throughout the maturation and the sexual cycle PROG remained unchanged in males, TEST was not altered in females, while E1 levels remained unaltered in both sexes.