The aim of this study was to clarify whether the increased vascular tone in spontaneous hypertension of rats is due to a change of the calcium-sensitivity of the contractile proteins themselves. In skinned rat tail artery rings from SHRSP and WKY rats the calcium-requirement for half maximal activation (3.6 X 10(-6)M Ca++ for both rat strains) as well as relaxation half times (1.45 +/- 0.43 min, SHRSP and 1.63 +/- 0.48 min, WKY) were found to be identical. The extent of myosin phosphorylation in the contracted and in the relaxed state did not differ between SHRSP and WKY. It is concluded that changes at the level of the contractile proteins are not involved in the increased vascular tone of SHRSP essential hypertension.