1. Insulin binding sites were characterized and quantified in snail (Helix aspersa) ganglia by incubation of tissue sections with 125I-porcine insulin, autoradiography with [3H]Ultrofilm, image analysis coupled to computer-assisted microdensitometry, and comparison with 125I-standards. Cellular localization was performed in the same sections by emulsion autoradiography. 2. Specific insulin binding sites were demonstrated in discretely localized groups of neurons of the cerebral, pleural, parietal, visceral, and pedal ganglia and in nerves. Scatchard analysis performed with consecutive sections from single animals revealed a single class of high-affinity insulin binding sites (Kd, 0.13 +/- 0.01 nM; Bmax, 157 +/- 10 fmol/mg protein). 3. Our results suggest that insulin may play a role as a neurotransmitter or neuromodulator in snail ganglia.