The projection areas of vitally stained motor end plates in normal and tenotomized fast and slow twitch muscles of the rabbit hind limb were measured. There was a significant reduction in the mean projection area of the motor end plates of the slow muscle after tenotomy. There was no significant change in the projection area of the FDL or gastrocnemius motor end plates after tenotomy. After tenotomy the deep (slow) component of the tibialis anterior showed an increase in the mean projection area of its end plates and so approached the faster superficial component in this respect. It appears therefore, that the altered pattern of activity of the slow motoneurons (which is known to occur after tenotomy) is associated with a change in the end plate size towards that normally found in muscles innervated by phasic motoneurons.