None of the brain peptides has been demonstrated to be present in significant amounts in the cerebellum1,2. The only effector substance known in the Purkinje cell, whose axon carries the integrated output of the cerebellar cortex, is the decarboxylated residue of glutamic acid3–6. An antiserum raised against synthetic porcine gut motilin and directed at the midportion and carboxyl end of the 22-amino acid polypeptide7 shows, in anatomically intact rat, mouse and human cerebellum, that most Purkinje cells contain motilin or a closely related substance8. We report here that motilin-absorbable immunoreactivity is evident from the spines of tertiary dendrites, through the cell body and axon, to nerve endings terminating, as expected, in the deep cerebellar nuclei and, unexpectedly, on nerve cells of some brain stem nuclei. In addition to its obvious functional implications, motilin is thus an important marker with which to re-examine the organization and projections of Purkinje cells, their ontogeny and their response to injury.