1. Field stimulation of desheathed preparations of guinea-pig vas deferens, treated with a ganglion-blocking agent, has revealed the presence of two tetrodotoxin-susceptible components in the motor response, suggesting the existence of two sets of post-ganglionic motor nerve fibres of different excitability: one set responding maximally to pulses of 0·1-0·4 msec; the other, to pulses of 2 msec. No distinction could be made pharmacologically between the two components.