Motilin-like immunoreactivity (MLI) was determined in cerebelli and pituitaries of the rat from 4 days before birth to about 6 months after. Cholecystokinin (CCK)-like immunoreactivity was measured in the same pituitary samples for comparison. MLI concentration was highest 4 days prenatally in both tissues, while total tissue MLI content was higher after birth. Cerebellar MLI content was maximal from day 14 to 40, while pituitary MLI content increased gradually with age, roughly paralleling the increase in tissue protein. Pituitary CCK was detectable only postnatally; its concentration was maximal between days 19 and 29, and decreased substantially in older animals. The presence of detectable MLI in pituitary before birth is consistent with the observation that MLI can be visualized by immunocytochemistry in anterior pituitary somatotrophic cells, which are known to be active producers of growth hormone before and after birth. The presence of MLI before birth in cerebellum is consistent with the localization of MLI in Purkinje neurons, known to be formed before birth. The presence of MLI in the cerebellum before birth and its marked increase in content during the period of synaptogenesis are suggestive of a possible role of motilin in development.