The concentration of monoamines and/or their metabolites has been measured using in vivo microdialysis in the ventromedial and lateral hypothalamus following the parenteral administration of insulin. Similar measurements were made in rats with lateral hypothalamic microdialysis cannulas in which insulin was given but food intake delayed for 6 h. Serum norepinephrine and epinephrine both increased following injection of insulin, and food intake rose after a delay of 30-60 min. Norepinephrine rose in both the ventromedial and lateral hypothalamus with peaks at 60-90 min. 3-Methoxy-4-hydroxy-phenylglycol (MHPG) and 3,4-dihydroxyphenylacetic acid (DOPAC) increased in the dialysis from the lateral but not the ventromedial hypothalamus. Serotonin was unchanged following the injection of insulin, but its metabolite, 5-hydroxy-3-indole acetic acid (5-HIAA), fell gradually in both areas. In animals which had recovered from the hypoglycemia during the 6 h after insulin injection, but had not been allowed to eat, the concentration of norepinephrine, serotonin, MHPG and 5-HIAA were all increased in the lateral hypothalamus. DOPAC, however, showed no elevation. With the beginning of food ingestion, all monoamines and their metabolites fell to or toward normal within 30-60 mins. These data are consistent with the hypothesis that glucoprivation may involve increased turnover of norepinephrine and serotonin. Changes in DOPAC, however, appear to be related to the 'stress' components rather than the food intake.