The occurrence of collateral and ultraterminal sprouting by skeletomotor and fusimotor axons is described in teased, silver preparations of normal and de-afferentated hindlimb skeletal muscles. In twelve cat and ten rabbit muscle spindles the proportions of plate-ending fusimotor axons showing sprouting were 30$\cdot $4 and 34$\cdot $0%, respectively. If small end-plates, previously described in the literature as 'accessory endings', are regarded as young plates newly formed by collateral sprouts, the degree of sprouting in the two spindle samples is increased to 33$\cdot $9 and 38$\cdot $3%, respectively. Sprouting by skeletomotor axons was observed in a variety of cat, rabbit, and rat muscles taken from normal muscles. In a sample of 567 terminal branches examined in muscles from the three animals, 8$\cdot $1% bore sprouts, or 19$\cdot $4% if the collaterals forming accessory endings are included as sprouts. Some evidence of retrograde degeneration among the terminal branches of skeletomotor axons innervating normal muscle is described. It is suggested that sprouting effects the replacement of old end-plates which degenerate after a limited life-span; and that it is a general property of the vertebrate motoneuron for its peripheral terminals to undergo cyclic renewal in this way. The greater degree of sprouting shown by fusimotor plate-ending axons as compared with skeletomotor ones is attributed to the fact that the innervation supplied to spindles by such axons is both multiple and polyneuronal. In conclusion, some of the implications of the replacement hypothesis, e.g. with regard to neuromuscular pathology, are discussed.