The effect on the KCl-induced contraction in estrus cycle stages of normal and ovariectomized rats using isolated rat thoracic aortic rings is presented for the first time.  The maximal contraction force exerted in the four stages of normal cycle was similar. The results suggest that in the contraction, the similarity in the effects obtained at different stages of the estrus in the rat, go together with a similar amount in voltage-operated calcium channels. By contrast, in ovariectomized rats, the contraction was different. It is observed a decrement in voltage-operated calcium channels in ovariectomized rats (OVX) at day 8 with respect to OVX at 48 hours and normal rats. The explanation may be in relation to nonspecific changes in the medial structure of the artery in relation to the number of voltage-operated calcium channels, due possibly to alterations in calcium metabolism following ovariectomy.Key words: KCl, estral cycle, ovariectomy, tonic contraction, calcium