This study explored the prejunctional actions of vecuronium using thein vivocat soleus nerve-muscle preparation. Vecuronium doses of 1 to 5 μg/kg iv suppressed the repetitive firing of the motor nerve endings, and the obligatory potentiation of the twitch response following high-frequency conditioning at 400 Hz for 10 s without attenuating neuromuscular transmission at 0.4 Hz. It was also found that extremely low doses of vecuronium had excitatory effects at the cat soleus motor nerve endings: doses of 0.5 and 1 μg/kg is evoked a modest postdrug repetitive firing of the nerve endings and a concomitant potentiation of the muscle responses. These dose-related agonist and antagonist activities suggest that at the motor nerve endings, vecuronium is a weak partial agonist. The major action of vecuronium at the motor nerve endings, however, was suppressive, and this antagonist action contributed to the neuromuscular blocking action of this muscle relaxant.