In male Wistar rats the influences of age and experimental obesity on the activity of malic enzyme (EC 126.96.36.199) in different organs were studied. Obesity was induced in newborn rats by injection of Na(+)-L-glutamate (2 mg/g b.w. daily) subcutaneously in the first 5 days. The enzyme activity was measured at the ages of 2, 6 and 18 months. In control animals the highest enzyme activities were found in the heart muscle, liver, epididymal fat pad and skeletal muscle after 6 months. After 18 months the activities in these organs are considerably reduced. In the kidneys the activity between the 2nd and the 18th months tends to decrease continuously and only the brain shows an opposite trend. In comparison with the control animals, in glutamate treated rats the enzyme activity doubles nearly in the lipogenic organs liver and fat tissue in all age groups. In liver and fat tissue of 6-month-old rats, previously treated with clonidine to stimulate growth hormone secretion, the activities are lower than in glutamate obese rats without clonidine, but still higher than in normal control animals. The qualification of glutamate obese rats as a model for the study of age-associated diseases like obesity or diabetes mellitus type II needs further investigation.