1. The effect of tetanus toxin on neuromuscular transmission in the abductor superficialis muscle of the goldfish fin has been investigated.2. The abductor superficialis muscle is multiply innervated and junction potentials (j.p.s) and miniature junction potentials (min.j.p.s) can be recorded with an intracellular micro-electrode at any point of impalement. Intracellular recordings have been made from muscles in which neuromuscular transmission has been blocked, either completely, or partially, by I.M. injection of tetanus toxin. In addition, the tension response of both acutely and chronically toxin-blocked muscles to carbachol (1 x 10(-4)M) has been investigated.3. As the neuromuscular block proceeds, the frequency of min.j.p.s falls, and some time after the muscle has stopped responding to nerve stimulation the min.j.p.s disappear.4. In muscles in which the block has not yet proceeded to completion, it has been found that the reduction in the frequency of the min.j.p.s is unaccompanied by any change in their range of amplitudes.5. The min.j.p. frequency can be greatly increased in such incompletely blocked preparations by repetitive stimulation of the nerve (at, for example, 100/sec for 10 sec). The min.j.p.s obtained are indistinguishable from those seen in the absence of stimulation. Additionally, min.j.p.s can be evoked by similar repetitive stimulation in muscles which are completely blocked, and in which no min.j.p.s are seen without stimulation, so long as the block has only been complete for a short time.6. The tension response to carbachol of muscles which have been paralysed by tetanus toxin for only a few days is identical with that of normal muscles. In contrast, chronically toxin-paralysed muscles contract more rapidly, usually more vigorously, and relax more rapidly than normal muscles.7. It is concluded that tetanus toxin prevents both the nerve-stimulated and spontaneous release of acetycholine from the presynaptic terminals in the abductor superficialis muscle of the goldfish fin.