1. The electrical and structural characteristics of ;slow' muscle fibres of the frog were studied in normal and denervated muscles, and in muscles undergoing re-innervation by a mixed nerve containing large and small motor axons.2. In agreement with previous studies, slow fibres in normally innervated muscles were incapable of producing action potentials.3. Approximately 2 weeks after the sciatic nerve was transected or crushed, slow muscle fibres acquired the ability to generate action potentials. These fibres were positively identified as belonging to the slow type, because their passive-electrical and ultrastructural characteristics remained essentially unchanged after the operations.4. The action potential mechanism induced in slow fibres is sodium-dependent, and is blocked by tetrodotoxin.5. After long-term re-innervation by a mixed nerve, slow fibres lose their acquired ability to generate action potentials, presumably because small motor axons re-establish connexion with the fibres.6. It is concluded that the action potential mechanism of slow muscle fibres is under neural control, and is normally suppressed by small motor axons.