The tonic contractions which are extremely resistant to removal of the external Ca were investigated in the rat vas deferens and myometrium. Both the noradrenaline response in the vas deferens and the oxytocin response in the myometrium could be repeatedly produced without appreciable diminution in Ca-free solution for more than 24 hrs. On the other hand, the tissue Ca content decreased exponentially after Ca-removal with a half time of 130-180 min. When Ca was readmitted, no indication of the early transient contraction was observed in the subsequent response in Ca-free solution, but the response was reduced compared with the response before Ca readmission. Verapamil suppressed the response in the presence of Ca, while it had very weak inhibitory effect even at 10 microM. Calmodulin antagonists of phenothiazine derivatives had a strong inhibitory effect on Ca-induced contractions, whereas they had very weak effects on the receptor-mediated contraction in Ca-free solution. Another calmodulin antagonist, W-7 suppressed both Ca-induced contraction and the contractions independent of external Ca. HA-1004, a vasodilator which has a structure similar to W-7, reduced the receptor-mediated contraction in Ca-free solution without much effect on Ca-induced contractions. These results may suggest that the receptor-mediated contractions resistant to Ca-removal are caused by some process without a contribution of the Ca-calmodulin system.