In the denervated extensor digitorum longus muscle of the rat type I and type II muscle fibres were differentiated histochemically and their course of atrophy was studied. Until 42 days after denervation type I and type II fibres could be identified by means of the myofibrillar ATPase reaction. Up to that time an exclusive atrophy of type II fibres was found. Type I fibres, the smallest of the normal muscle, did not change their diameters and therefore represented the largest fibres 42 days after denervation. Type II fibres of the "white" muscle portion, in which the larger IIB fibres are predominant, showed a higher rate of atrophy than those of the "red" muscle portion, in which the smaller IIA fibres are predominant: by 42 days the diameters of all type II fibres had gone down to equal values. Combined with a further progress of atrophy at later stages, there was a dedifferentiation of the histochemical properties, and the type I fibres exhibited atrophy as well. 120 days after denervation all muscle fibres were found to be highly atrophied.