The pharmacokinetics of free and total valproic acid (VPA) in plasma and whole blood after oral administration during steady state was investigated in seven infants (mean age 10.7 months) receiving monotherapy. The VPA concentrations in whole blood closely followed those in plasma but at a reduced level. A positive correlation was found between dose and mean plasma concentration (r = 0.71). Mean terminal half-lives were similar in plasma and whole blood (12.5 and 15.5 h, respectively), but were considerably longer than for free VPA (6.4 and 6.5 h, respectively; P less than 0.01). There was a significant decrease in half-lives with increasing age (P less than 0.05). Plasma and whole blood clearance for total VPA was higher than reported in older infants and adults (17.8 and 28.9 ml/kg per hour) and was considerably higher for free VPA (127.6 and 188.8 ml/kg per hour, respectively). The increase in clearance compared with that in older subjects is well in concordance with a lower protein binding of VPA (mean 85.3%). Of special importance is that the percentage of unbound VPA increased with increasing concentrations of total VPA. The fraction of unbound VPA in plasma increased even more in subjects with low albumin concentrations (P less than 0.01).