1. Membrane fluidity of erythrocytes and cultured vascular smooth muscle cells was studied in spontaneously hypertensive rats (SHR; Okamoto and Aoki strains) by means of an electron spin resonance (e.s.r.) and a spin-label technique.

2. The values of outer hyperfine splitting (2T) and of the order parameter (S) determined from e.s.r. spectra for a fatty acid spin-label agent (5-nitroxy stearate) were significantly higher in erythrocytes of SHR (4 and 10–13 weeks of age) than in those of age-matched Wistar–Kyoto rats (WKY). This finding suggests that the membrane fluidity of erythrocytes might be decreased in SHR.

3. The fluidity of cultured vascular smooth muscle cells obtained from 10-13-week-old SHR was also reduced compared with that from WKY.

4. These results suggest that the decreased membrane fluidity might be a genetically determined abnormality of the cell membranes in SHR.

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