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. 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. 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. 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. 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.