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The effect of type C1 botulinum toxin （BoTx） on neuromuscular junctions was investigated using tibial nerve-soleus muscle preparation of rats. Sublethal doses of purified type C1 BoTx were injected subcutaneously into the postero-lateral region of the right lower hind limb. At 1, 3, 5, 7 and 10 days after the injection, the preparation was excised and analyzed in vitro for blockade of neuromuscular transmission. Analysis of alterations in miniature end-plate potentials （m. e. p. p. s） and end-plate potentials （e. p. p. s） was also carried out using a preparation of 0.25 mouse intraperitoneal LD50 at 1 day after injection, to avoid the complication of denervation-like effects. The following results were obtained. 1. A dose-dependent blockade of neuromuscular transmission was induced by type C1 BoTx. The recovery from the blockade also showed a dose-dependency. 2. The mean quantal content, the amplitude of e. p. p. s, and the amplitude and frequency of fast m. e. p. p. s in BoTx-poisoned preparations were significantly lower than those in unpoisoned controls. In addition, failure of e. p. p. s was frequently observed in BoTx-poisoned preparations. 3. Type C1 BoTx did not affect the shape of the amplitude histogram for fast m. e. p. p. s. 4. There were no significant differences between the amplitude and frequency of slow m. e. p. p. s in BoTx-poisoned and unpoisoned preparations. 5. The acetylcholine sensitivity of postsynaptic membrane and the magnitude of resting membrane potential of muscle fibers were uninfluenced by type C1 BoTx. From these results, we concluded that the blockade of neuromuscular transmission induced by type C1 BoTx was attributable to the inhibition of Ca++-sensitive quantal release from the motor nerve terminals.