The effectiveness and toxicity of phenobarbital (PB), primidone (PRM), and sodium valproate (VPA), used exclusively in monotherapy, were compared in 95 children affected with febrile convulsions. Treatment was restricted to either complicated or simple febrile convulsions with risk factors. The effectiveness and toxicity of each drug were related to the daily dose and the steady-state plasma levels. PB (4.8 +/- 0.7 mg/kg/day) achieved plasma levels of 16.4 +/- 2.8 micrograms/ml and prevented febrile convulsions in 80% of the patients. Side effects were observed in 76.7% of the patients, a change in dose being required only in 13.3%. PRM (17.8 mg/kg/day) yielded PB plasma levels of 14.1 +/- 3.7 micrograms/ml and was effective in 88.2% of the patients. The incidence of side effects was 53%, but no change in treatment was required. VPA (35.2 +/- 5.9 mg/kg/day) achieved plasma levels of 57.2 +/- 15.3 micrograms/ml (measured before the first dose in the morning) and was effective in 91.7% of the patients. Side effects were detected in 45% (significantly lower than after PB, p less than 0.01), and required a change in treatment in 14.3%. No differences in doses and plasma levels were found between patients with or without recurrence of febrile convulsions and with or without side effects; an exception was the higher doses of VPA administered to patients who showed side effects. It is concluded that PRM and VPA were at least as effective and well tolerated as PB. Because the plasma levels of the three drugs were near the lower limit of the therapeutic range, it remains to be elucidated whether higher doses may increase the benefit without adding unacceptable toxicity.