1. By using a direct method of recording activity the diurnal rhythm of activity of Periplaneta americana L. has been investigated in detail under various conditions of light and darkness.

2. It has been found that there is no direct connexion between a hunger cycle and the activity rhythm.

3. By means of experiments in which cockroaches were joined together in parabiosis it has been shown that a rhythm can be relayed from one insect to another, and is postulated that a secretion carried in either the blood or tissues is responsible for the transmission.

4. Implantation of sub-oesophageal ganglia from cockroaches with a known rhythm into headless cockroaches has been found to result in the appearance of the known rhythm of activity in the latter.

5. Effects of other endocrine organs on the activity rhythm has been investigated.

6. The ocelli have been found to be directly connected with the establishment of the rhythm by the external factors of light and darkness.

7. A rhythm of activity subsidiary to the main diurnal rhythm is described.

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