Plasma concentrations of primidone and its metabolite phenobarbitone were monitored in 9 pregnant epileptic patients treated with primidone (and in 3 cases other antiepileptic drugs) given at constant doses throughout pregnancy and the puerperium. Phenobarbitone plasma concentrations were monitored in another 6 patients given phenobarbitone itself. A trend towards increasing primidone plasma concentrations during the second quarter of pregnancy was evident in all patients, with a concomitant significant decrease in primidone-derived phenobarbitone plasma concentrations. A trend towards a lowering of plasma concentrations of phenobarbitone administered as such was confirmed.These results suggest the usefulness of a careful monitoring of primidone and primidone-derived phenobarbitone during pregnancy and the puerperium. Discrepancies of findings with primidone and phenobarbitone are discussed in view of the possible mechanism involved.