Immunoreactivities for tyrosine hydroxylase (TH) and neuropeptide Y (NPY) were studied in developing sympathetic neurons of bullfrog tadpoles and adults. At stage III, nearly all ganglion cells are positive for TH. This suggests early commitment to an adrenergic phenotype, the timing of which is analogous to that reported for sympathetic neurons in birds and mammals. During metamorphic stages and in juvenile bullfrogs, the expression of TH becomes transiently bimodal: many neurons are intensely positive; the remainder are faintly positive. In adult sympathetic neurons, TH expression is more uniform. NPY first appears in a few principal neurons (less than 1%) of paravertebral ganglia 9 and 10 at stage XI. The percentage of ganglion cells containing NPY then increases gradually, reaches adult levels (approximately 55%) by stage XX, and persists at these levels through metamorphosis. The development of NPY expression follows a similar time course in paravertebral ganglion 6. Double-label experiments in late- stage tadpoles and juvenile bullfrogs revealed that the intensely TH- positive neurons are negative for NPY. Taken together with recent electrophysiological data (Horn and Stofer, 1990), these results demonstrate that the development of NPY expression begins long after the onset of adrenergic differentiation and ganglionic synapse formation. The present findings also show that cellular levels of TH and NPY can be independently altered, and they suggest that the onset of NPY expression is not linked to maturation of peripheral targets, but rather to some more global event operating synchronously along the rostro-caudal axis.