According to the staining intensities for AChE, the motor end-plates of the rat diaphragm can be classified into strong (S) and pale (P) types. About 34% of the total end-plates of the rat diaphragm are of S type and 50% of P type. The P end-plates differ from S end-plates in two aspects. First, the secondary subneural clefts of the S end-plates are well developed. They are numerous, long, closely packed and often branched. On the other hand, the secondary subneural clefts of the P end-plates are short, sparse and usually unbranched. Secondly, there seems to be a variation in AChE activity in the P end-plates. Focal negative AChE areas are found in the subneural apparatus of some P end-plates. It is concluded that the less well developed secondary subneural clefts and focal areas of negative AChE activity contribute to the paler staining of the P end-plates.