1. As shown previously, slow muscle fibres of the frog develop the ability to produce action potentials in 2 weeks after denervation.2. Normal transmission at the slow fibre neuromuscular junction gives a junction potential of 11 mV average (range 6-20 mV), caused by summation of potentials from several motor axons.3. Botulinum toxin injected intramuscularly into the iliofibularis blocks the neuromuscular junction of slow fibres in 6 days at room temperature. Single nerve stimuli give junction potentials of 0.05 mV average (range 0-1.6 mV). Contraction of slow fibres in response to tetanic stimulation is eliminated.4. No action potentials could be elicited in botulinum treated fibres, even 6(1/2) weeks after injection of toxin. This finding is discussed in relation to the possible involvement of a ;trophic' factor regulating the action potential mechanism in frog slow muscle fibres.