SUMMARY: Prolactin, injected into ring doves with previous breeding experience, is relatively ineffective, compared with progesterone, in inducing incubation behaviour. Even at dosage levels inducing full crop growth, only 40% of the individuals tested, coming from 65% of the pairs tested, sat on eggs. This is significantly less than the response to progesterone, which induces incubation behaviour in 70% of the individuals, representing 95–100% of the pairs tested. At smaller dosage levels of prolactin, it was possible to cause significant increases in crop weight by injecting the hormone in amounts having no effect whatever on incubation behaviour. Since incubation normally starts well before the onset of crop growth, it appears that incubation normally begins when blood prolactin levels are too low to induce it. We therefore conclude that prolactin is normally not principally responsible for the initiation of incubation behaviour, at least in the ring dove.