Neurotransmitters and neuropeptides coexpressed by the same neuron may be regulated independently. The peptide transmitter, neuropeptide Y (NPY), has been identified in approximately 60% of rat sympathetic superior cervical ganglion (SCG) neurons, virtually all of which are catecholaminergic. However, we find that environmental signals which affect noradrenergic traits in cultured rat SCG, regulate NPY expression differently. Relatively large increases in neuronal density only slightly alter catecholaminergic properties in sympathetic neurons. However, levels of NPY were highly dependent on cell density. In pure neuronal cultures, increasing cell numbers from 8000 to 12,000 neurons per culture resulted in a 10-fold decrease in NPY. Coculture of SCG neurons with ganglion nonneuronal cells further reduced NPY levels at all neuronal densities examined. Treatment with the neuropoetic cytokine, leukemia inhibitory factor, which increased the expression of cholinergic traits and decreased noradrenergic expression, decreased NPY by 50% in cocultures. Finally, neither glucocorticoids nor the cytokine interleukin-1 beta, which regulate other transmitter systems, altered NPY expression. These observations indicate that although noradrenergic traits and NPY are often coexpressed, they are regulated independently in SCG neurons. Further, expression of NPY is highly influenced by cell-cell contact.