1. The intracellular sodium content and the sodium efflux rate constant have been determined in vitro in thymocytes derived from the Okamato-Aoki strain of spontaneously hypertensive rats.
2. A strong positive correlation between the systolic blood pressure and the sodium content of thymocytes was observed (r = 0.59, n = 39, P < 0.001).
3. The rate constant for total sodium efflux was negatively correlated with systolic blood pressure (r = −0.43, n = 45, P < 0.005) and this was due to a fall in the ouabain-sensitive component of sodium efflux.
4. Sodium efflux, influx and the thymocyte potassium content were not related to the blood pressure.