The concentration of free GABA was measured in the plasma of 18 normal young men, in three separa te studies. Overall, it varied by a factor of about 4-fold in normal subjects, from values of 0.10 nmol/ml to 0.40 nmol/ml. Day-to-day stability was found to be very high (r=0.94, n=8 subjects, p<0.002). Free GABA concentration was stable over more than 2 weeks (r=0.95, n=5 subjects, p<0.02). A study of circadian rhythms was carried out (n=5 subjects) comparing a condition with no food and partial light deprivation in a supine position during 34 hr. ("rest" condition") with a condition of normal daily activities ("activity" condition). No circadian rhythm was found in the "rest" condition. In the "activity" condition, there was a mid-day rise in GABA concentrations in 3 out of 5 subjects. The significance of these changes was explored using different statistical tests.