Neurochemical changes in the ventromedial hypothalamus (VMH) after a single intravenous injection of streptozotocin were examined, using in vivo brain microdialysis under free-moving conditions. Although streptozotocin-induced diabetes produced significant decreases in extracellular concentrations of noradrenaline (NA), serotonin (5-HT), and their metabolites in the VMH, the ratios of 3-methoxy-4-hydroxyphenylglycol/NA and 5-hydroxyindoleacetic acid (5-HIAA)/5-HT were increased. Experimental diabetes led to a pronounced increase in extracellular GABA, which correlated strongly with the decrease in dialysate levels of NA, and to a smaller extent with that of 5-HT. A modification of dopamine (DA) metabolism was induced in the VMH of diabetic rats, whereas there was no change in dialysate DA levels. Daily injections of insulin were able to restore their levels to normal in the areas tested in the microdialysis study. The equal increases in dialysate 5-HT and 5-HIAA and the better restoration of the 5-HIAA/5-HT ratio after insulin therapy indicate that serotonergic activity may depend on the levels of circulating insulin more than on noradrenergic activity. Circulating NA was reduced in streptozotocin-diabetic rats, suggesting that the diabetes-induced reduction in sympathetic activity is accompanied by decreases in NA, or 5-HT, or both, in the VMH.