The effects of Cd2+ on the skinned taenia coli were compared with that of Ca2+ channel blocker, D-600 and calmodulin inhibitor, chlorpromazine. The Cd2+ (0.001-0.5 mM) inhibited the contraction of skinned fibres in a dose-dependent manner. In contrast, the contraction of skinned fibres was not inhibited by pretreatment with 1 X 10(-6) M D-600. However, chlorpromazine (10(-4) M) inhibited the contraction of skinned taenia coli. The inhibition of contraction by Cd2+ in skinned taenia coli was completely reversed by washing with solution containing EDTA, following a treatment with 0.5 mM Cd2+ for 15 min. The skinned fibres had rapidly taken up a far greater amount of Cd2+ than intact muscle of taenia coli. EDTA increased Cd2+ efflux in skinned fibres and only 5% of the original tissue content retained. Thus, EDTA, which was ineffective on intact taenia coli, can be considered to act on Cd2+ of intracellular compartment after disruption of the cell membrane with saponin. In conclusion, there is the possibility that Cd2+ inhibits the contractile system, such as contractile proteins and calmodulin, which results in tension inhibitory effect.