By using hormone preparations, a simulated migratory behaviour has been induced in caged Z. l. gambelii, a nocturnal migrant. Daily injections of prolactin increase nocturnal restlessness in photosensitive birds under outdoor conditions prior to spring migration and also initiate nocturnal locomotor activity in photo-refractory birds held indoors on a long daily photoperiod. These effects are augmented by adrenocortical hormones which, however, are ineffective when administered alone. In addition, metapirone, an inhibitor of adrenocortical synthesis, suppresses nocturnal activity. Simultaneous injections of corticosterone eliminate the metapirone-induced suppression of nocturnal activity. In prolactin-treated birds, there is a reduction of locomotor activity during the last two hours of light that is strikingly similar to the late afternoon reduction in activity as natural Zugunruhe developes. The dual role of prolactin in promoting fattening and nocturnal locomotor activity, and the augmentation of nocturnal activity by adrenocortical hormones have been discussed in regard to their possible involvement in the physiologic events of migration. Assays of pituitary prolactin at several times during the annual cycle appear to be compatible with a hypothesis involving prolactin in the development and maintenance of migratory behaviour.