We turned our attention to an examination of guanylyl cyclase in cell-free systems. We learned that there was enzyme activity in both high-speed supernatant and particulate fractions of most tissue homogenates. Furthermore, the activities were quite different. While ATP inhibted both activities, the soluble activity was more sensitive to ATP inhibition. Calcium ion could either activate or inhibit activity dependent upon its concentration, but the soluble and particulate activates had different sensitive to these effects.