1. A study was made of the concentration of potassium and sodium in the blood of Periplaneta americana to determine whether daily fluctuations occur which might imply ionic control of the circadian locomotor activity rhythm.

2. Analyses were carried out on 166 blood samples taken at different times of day. A fall of about 10% occurred in the potassium concentration during the first hour of darkness; there was also a gradual decline of 2% in the sodium concentration.

3. However, series of six successive blood samples taken over a period of 3 weeks from ten individual cockroaches revealed no daily change in the level of potassium, sodium or haemocyte density.

4.No conclusions are drawn on the activity control aspect, but the results do suggest that daily changes may occur in cockroach blood ions.

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