Sustained-release compositions for delivering therapeutic concentrations of isovaleramide, isovaleric acid, and certain structurally related compounds are provided for the treatment for a variety of pathological conditions, including epilepsy and spasticity, which are ameliorated by effecting a modulation of CNS activity. The ability of the compositions to sustain relatively constant levels of the drug at a therapeutic dose in the serum for extended periods of time enables a once or twice daily administration schedule.