Morphometric studies conducted on the blood vessels of the spontaneously hypertensive rat have provided evidence that medial hypertrophy is a key characteristic of the vascular change which occurs in hypertension. In the present study, we determined whether 3-methylhistidine (3MH), a post-translationally modified amino acid which is found uniquely in the actin and myosin of muscle, could provide a biochemical marker of such change. Our results indicated that the concentrations of 3MH were selectively elevated in the blood vessels from the spontaneously hypertensive rat, when compared with concentrations in vascular tissues from the Wistar-Kyoto rat. The concentrations of 3MH in non-vascular tissues were similar in the two strains. Chronic captopril treatment prevented the development of hypertension in the spontaneously hypertensive rat and was associated with a reduction of the vascular concentrations of 3MH. We therefore conclude that blood vessel concentrations of 3MH are a useful biochemical index of the changes in vascular smooth muscle contractile protein which occur during the development of hypertension in the spontaneously hypertensive rat.