Six putative neurotransmitters and the beta-receptor excitant isoproterenol were applied to bulbar respiratory neurons classified according to their burst discharge in the respiratory cycle and, for comparison, also to "unspecific" cells. With glycine inhibition occurred in I and prevailed in unspecific neurons. With GABA inhibition preponderated in I and E neurons. Glutamate excited E neurons. With dopamine inhibition prevailed in I and unspecific neurons. The majority of E neurons remained unaffected. With NE excitation occurred in EI and preponderated in I, IE, E and unspecific neurons, while some IE and E neurons were inhibited. The effects of isoproterenol did not allow any clear statement about receptor properties. In I units, however, activation was more frequent than inhibition. With 5-HT excitation prevailed in IE neurons. About half of the I cells remained unaffected and in the remainder inhibition preponderated over activation. This suggests the existence of two 5-HT-specific receptors in I and IE neurons. Comparison of the single effects revealed differences in the receptor properties of the various cell types. Results suggest that some cell types possess dopaminergic receptors and that these differ from NE-receptors, which have been found in all cell types. NE-receptor stimulation apparently can result in neuronal activation or inhibition.