Catecholamine-induced vascular smooth muscle contration is enhanced in female animals and in presence of emale sex hormones in vitro. Androgens appear to depress these responses. Sex steroids may also alter calcium ion (Ca++) binding and metabolism. We compared contractility as well as quantity and relative lability of tissue calcium pools in male and female rat isolated aortic strips bathed in Ca++-free solution. We also studied aortic strips from 21-day postpartum lactating female rats to determine the effects of previous high circulating levels of female sex steroids (present during pregnancy) and prolactin (present during lactation). Rat aortic strips were found to contain loosely and more tightly held calcium stores. Strips from males were unresponsive in Ca++-free solution unless previously exposed to a Ca++-rich bathing medium. They accumulated more tissue calcium when bathed in Ca++-rich solution than did strips from females. This extra calcium appears to reside in the loosely-held fraction. Tissues from males first incubated in Ca++-rich solution to enhance the loosely held fraction respond more readily in Ca++-free solution to a high potassium (K+) concentration than to epinephrine. Strips from females respond about equally to high K+ or epinephrine whereas aorta from lactating female rats are much more responsive to epinephrine in Ca++-free solution and gain less calcium in Ca++-rich medium than those of the other rats. These data suggest that in the presence of high circulating levels of female sex hormones or other female factors (e.g., prolactin) enhanced binding or sequestration of potential activator ions occurs which increases the responsiveness of the tissue to catecholamines. Male sex hormones and/or factors promote the capacity of the rat aorta to gain a more loosely held calcium fraction which is readily used for contraction by K+ depolarization.