Arterial blood pressure has been determined using a tail cuff method in 164 unanaesthetized, adult mice with genetically low (MGL) or high (MGH) erythrocyte magnesium levels. The mice came from the 9th, 10th, 11th and 12th generations of a bidirectional selective breeding undertaken at the CSAL-CNRS (Orleans). The two lines differed significantly by the magnesium contents of their erythrocytes, plasma, kidney and bone. Ten successive measurements of systolic blood pressure were recorded from each animal, without habituation, within a single period of about 2 min. All mice had elevated blood pressures presumably due to the stress of the experimental procedure. The first, the tenth and the average values of these 10 measurements yielded similar results in both sexes and in both lines. Younger animals (4 months of age) had significantly higher values (180 mmHg) than older ones (10-13 months, 161 mmHg), and this difference was more pronounced in the MGL than in the MGH strain. In both age-groups and lines, about two-thirds of all mice tested showed an increasing arterial pressure during the testing period while the remaining third exhibited decreasing values. Whatever the age-group or the variation pattern during the course of the blood pressure measurements. MGL mice had median values (mean of the fifth and sixth measurements) higher than those of MGH mice. The difference observed between the two strains can be attributed to a greater sensitivity and/or reactivity of the MGL mice to the stress induced by the manipulation, heating and immobilization required for blood pressure measurement.