1. Pyriformis muscles of Rana temporaria were denervated by crushing the sciatic nerve inside the pelvis. At different times during regeneration of the nerve, slow muscle fibres were examined for the presence of all-or-none responses. 2. During the early period of re-innervation (35-60 days), slow muscle fibres were found to be non-selectively re-innervated by (foreign) fast-conducting motor axons. All slow fibres during this period were able to produce full sized action potentials, with overshoot. 3. Action potentials markedly decreased in amplitude and eventually disappeared completely between day 61-110 following denervation; during this period, slow muscle fibres were re-innervated by slowly-conducting motor axons. 4. Functional re-innervation by slowly-conducting motor axons, in the late stages of re-innervation, was not a necessary condition for the suppression of the action potential. Slow fibres innervated by fast motor axons, as well as denervated slow fibres, lost the action potential simultaneously with slow fibres which were re-innervated by slowly-conducting motor axons. 5. It is suggested that (small) slowly-conducting motor axons can exert a 'trophic' influence on the slow fibre membrane, independent of their synaptic function.