The pharmacokinetic properties and relative bioequivalence of two formulations of the antiepileptic Epilimm were compared in a three-period, repeat dose, randomized crossover study in 18 male volunteers. A daily dose of 1000 mg of antiepileptic was given either as twice-daily entericcoated sodium valproate tablets or as twice-or oncedaily controlled release tablets (EpilimB Chrono, sodium valproate/valproic acid mixture). All regimens were bioequivalent with respect to area under the curve and elimination half-life. The twice-daily controlled release formulation showed reduced mean peak plasma valproate levels and raised mean trough levels compared with the enteric coated tablets. The once-daily controlled release regimen gave reduced mean peak levels but similar mean trough levels to the twice-daily enteric-coated regimen. No major differences between regimens were observed in the time at which peak concentration was observed. Both formulations were well tolerated. The most frequently reported adverse event was mild diarrhoea (all on Chrono treatment) which is probably related to the lack of enteric coating on this formulation. The reduced mean peak-trough variation in plasma valproate levels observed with the twice-daily controlled release regimen is likely to reduce further the low incidence of concentration-related side-effects with sodium valproate and permit more reliable plasma level monitoring. This study also indicates that once-daily treatment with Epilim" controlled release would be a suitable replacement for twice-daily dosing with either formulation. Once-daily treatment is likely to give considerable benefits both in convenience and in better patient compliance.