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The effectiveness of progesterone (100 μg./day, × 7) in inducing incubation behaviour in male ring doves is markedly diminished by castration. Priming with testosterone propionate (200 μg./day, × 14) restored the effectiveness of progesterone in eliciting this behaviour to the levels in intact males. The synergistic relationship of these two hormones with regard to incubation behaviour contrasts with the inhibition by progesterone of an androgen-dependent male courtship display, the bowing-coo. The behavioural changes observed after these endocrine treatments are fully consistent with the normal sequence of changes in behaviour characterizing the transition from the courtship phase to the incubation phase of the reproductive cycle.