To explore the possible role of heat shock protein 20 (HSP20) -linked regulation of actin-myosin interaction in living vascular smooth muscle contraction, we studied the effects of HSP20p and TnIp, synthetic peptides originating from an actin tropomyosin binding region of human heat shock protein 20 [residues 110-121; GFVAREFHRRYR] and that of rabbit cardiac troponin I [residues 136-147; GKFKRPTLRRVR], respectively, on the active stress and phosphorylation level of myosin regulatory light chain (MLC₂₀) during relaxation of skinned (cell membrane permeabilized) preparations from "tonic" carotid artery and "phasic" taenia cecum from guinea pig. Active stress of the skinned preparations, resulting from actin-myosin interaction, biphasically decayed following Ca²⁺ removal (relaxation). Decay of MLC₂₀ phosphorylation level by Ca²⁺ removal was much faster than active stress in an exponential manner. In skinned carotid artery, HSP20p did neither affect relaxation time course nor MLC₂₀ dephosphorylation, whereas, in skinned taenia cecum, the peptide slowed relaxation time course through inhibition of MLC₂₀ dephosphorylation and slowing "latch"-bridge dissociation. On the other hand, TnIp accelerated relaxation time course without affecting MLC₂₀ dephosphorylation in both skinned carotid artery and skinned taenia cecum. Our present results suggest that, HSP20p slows the relaxation processes through intracellular regulatory mechanisms such as Rho A/Rho-kinase mediated pathways, which are known to be dominant in "phasic" smooth muscles but to be recessive in "tonic" smooth muscles.