The biochemical properties of the small, intensely fluorescent (SIF) cells of bovine and feline superior cervical ganglia were determined by studying the cyclic AMP responses to incubation in vitro with adrenergic agonists: the morphological properties were studied by fluorescence histochemistry and electron microscopy. Incubation with 50 micron dopamine elicited cyclic AMP levels 524% of control values in the bovine ganglion, but only 152% in the feline ganglion (a value which is not statistically significant). Incubation with 50 micron isoproterenol produced an increase of 356% of control values in the cow, but no increase in the cat. SIF cells were classified into two types by fluorescence histochemistry. The bovine ganglion contained 23.7% Type I (solitary) SIF cells, and 76.3% Type II (clustered) cells, whereas the feline ganglion contained 99.5% Type II cells. Thus the feline ganglion lacks both Type I SIF cells and a dopamine receptor-adenylate cyclase complex. On the basis of biochemical and morphological evidence, we hypothesize that the Type I SIF cell is an interneuron, and infer that the lack of a response to dopamine stimulation in the feline ganglion is caused by a scarcity of interneurons in this species.