Abstract The epidermis and the mucous gland of the thumb pad of male Rana pipiens were studied by histochemical and electron microscopic methods. During the breeding season the epidermis of the thumb thickens and becomes highly papillated while the mucous glands beneath increase enormously in size. The outer two-thirds of the epidermis reacts positively for alkaline phosphatase, acid phosphatase, alpha naphthol esterase, monoamine oxidase, succinic dehydrogenase and cytochrome oxidase. The mucous glands were found to consist of two different cell types. Most numerous are the typical secretory cells that give positive histochemical tests for alkaline phosphatase, acid phosphatase, alpha naphthol esterase but are unreactive for succinic dehydrogenase and cytochrome oxidase. Electron microscopic studies of these cells reveal typical secretory granules closely associated with the endoplasmic reticulum. The second cell type is nonsecretory that gives a strong positive reaction for succinic dehydrogenase and cytochrome oxidase and contains a rich complement of mitochondria.