1. We studied the properties of identified stomatogastric ganglion (STG) neurons grown in complete isolation in primary cell culture. 2. STG neurons isolated with a short piece of primary neurite adhered to the culture dishes and extended neurites. Outgrowth was apparent within several hours, and continued for < or = 5 days. 3. After 1 day in culture, most STG neurons were not capable of producing action potentials or oscillations. After 3-5 days in culture, most STG neurons regained the ability to fire action potentials, and some became endogenous bursters. Neurons in culture 3-5 days possessed many of the physiological properties of STG neurons in situ, including postinhibitory rebound, a hyperpolarization-activated depolarizing voltage sag, and the ability to burst in the presence of the potassium channel blocker tetraethyl-ammonium. 4. Identified cultured neurons responded appropriately to a variety of neuromodulators, including the monoamines dopamine and octopamine, the muscarinic agonist pilocarpine, and the peptide proctolin. These data suggest that the maintenance of receptor expression in fully differentiated STG neurons is not affected by isolation from all synaptic and modulatory influences. 5. In contrast to the other modulators tested, the effects of serotonin on cultured neurons differed from those reported in situ. Two cell types that are reported to be hyperpolarized by serotonin in situ, the lateral pyloric and pyloric neurons, were depolarized by serotonin in culture.