Feeding behavior, activity level, and thermoregulatory ability of mice made obese by neonatal administration of monosodium L-glutamate (MSG) were studied. The degree of obesity and other characteristics of the syndrome were found to depend on age, diet, and housing condition. Carcass fat determinations demonstrated the presence of obesity in all MSG animals; however, body weight was elevated over control levels only in adult mice caged in groups. Group-housed MSG animals also failed to increase food intake in response to food deprivation and were both hypoactive and hypothermic. INdividually caged MSG mice showed normal activity levels and body temperature, an attenuated response to food deprivation, and an enhanced response to a high-fat diet. Since MSG obesity may be the consequence of damage to the dopamine-rich arcuate nucleus of the hypothalamus, a second goal of the study was to measure central catecholamines and examined any changes in the MSG animal's behavioral responses to catecholaminergic drugs. Animals treated with MSG sustained some loss of hypothalamic dopamine, but no systematic relation between central catecholamines and behavioral aspects of the syndrome could be discerned.