1. The action of adrenaline and pituitary extract has been studied on the isolated heart of the crab Maia, the perfused heart of the bivalve Pecten, the isolated crop of the Gasteropod mollusc Aplysia, and the isolated pharynx of the annelid Aphrodite.

2. Methods for the study of the physiology of rhythmical contraction in these invertebrate preparations are described.

3. Pituitary extract in quantities comparable with those which excite the mammalian uterus was not found to have any action on invertebrate muscle.

4. Adrenaline (and epinine) in all cases produced a very pronounced increase in tone accompanied (in Maia, Aphrodite, and Pecten) with acceleration of the normal rhythm.

5. The specificity of the oxytocic action of pituitary extract and the parallelism between the action of adrenaline and of sympathetic excitation in vertebrates are discussed.

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