1. 1.

    A technique is described for continuous measurement, with ionselective microelectrodes, of K+ and Na+ activity in the blood of a free-walking cockroach Leucophaea maderae(Fab.). Measurements can be made for periods of up to 96 h.

  2. 2.

    In LD 16:8, there is a marked and consistent pattern in the diel variation of K+ activity, with minima 1 h before dusk and 2 h after dawn. The mean diel range in K+ activity covers a drop of 67 % below the maximum daily value. The maximum range in K+ activity for an individual cockroach in 24 h was from 4.5 to 25.0 mM-K+.

  3. 3.

    Simultaneous records of K+ activity and locomotor activity show that the first minimum in K+ activity occurs 1-2 h before the main, sharp peak in locomotor activity at dusk, and the second minimum 1-3 h after the subsidiary, broad peak in locomotor activity at dawn.

  4. 4.

    Diel fluctuations in whole-blood potassium concentration follow a roughly similar pattern to the K+ activity changes. Comparison of the concentration and activity changes allows some speculations about the diel movements of potassium within the blood.

  5. 5.

    Diel fluctuations in Na+ activity show no clear overall pattern.

Note: A.R.C. Unit of Invertebrate Chemistry and Physiology, Department of Zoology, Downing Street, Cambridge CBz 3EJ, England.

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