An increase in progesterone secretion has been claimed to play a significant role in mediating the transition from courtship to incubation behavior in male and female ring doves (Strep-topelia risoria). To test this idea, peripheral plasma progesterone concentrations were measured by radioimmunoassay throughout the reproductive cycle in male and female doves. In male doves, progesterone values averaged 1.27 ± 0.08 ng/ml (X ± SEM) plasma during the cycle. There were no significant changes detected i n progesterone concentration during the cycle in males. In contrast, progesterone concentrations increased significantly in female doves by the fifth day of courtship. Mean levels in incubating and brooding females averaged 1.13 ± 0.11 ng/ml, whereas on the fifth day of courtship progesterone levels rose to a mean of 3.01 ± 0.30 ng/ml. Further analysis showed that there was a significant positive correlation between progresterone concentration and follicular development and oviduct weight during courtship. No significant correlations between pro-gesterone level and follicular state or oviduct weight were found in females at other stages of incubation or brooding. It was concluded that although both the male and the female ring dove undergo a behavioral transition from courtship to incubation, only in the female is there a readily detectable increase in progesterone secretion correlated with this transition. (Endo-crinology94: 1547, 1974)