Effect of muscle spindle afferent invoked by intraarterial injection of succinylcholine (Sch) on respiration was assessed by changes in unit discharge of respiration related neurons and phrenic nerve of 63 anesthetized, bilaterally vagotomized and artificially ventilated rabbits. The results are as follows. (1) Intra~arterial injection of Sch prolonged inspiratory duration (Ti) or decreased expiratory duration (Te) and augmented respiratory frequency (RF). (2) 66% (37/56) of the medullary inspiratory neurons were excited and 10/17 of the expiratory neurons were inhibited by intraarterial injection of Sch, reaching maximum respectively at 1 2 and 2 3 min after injection, while non-respiration related neurons were not affected. (3) This effect of Sch was suppressed or abolished when muscle spindles were destroyed by intramuscular injection of bupivacaine.