The present studies were performed to determine whether the major biologically active metabolite of vitamin D3, 1,25-dihydroxycholecalciferol [1,25(OH)2D3], could influence the activities of rat colonic particulate guanylate cyclase and adenylate cyclase. To address these issues, colonocytes were harvested from Sprague-Dawley rats and suspended in Krebs-Ringer bicarbonate buffer. The cells were then treated with 1,25(OH)2D3 or other agents (see below) and crude membranes were prepared and analyzed for particulate guanylate cyclase and adenylate cyclase activities. The results of these studies demonstrated that: 1) 1,25(OH)2D3, in a concentration-dependent manner, rapidly (within minutes) stimulated guanylate, but not adenylate cyclase activity; 2) preincubation of the cells with staurosporine, a protein kinase inhibitor, or U73122, an inhibitor of phosphoinositide-phospholipase C-dependent processes, blocked the increase in guanylate cyclase activity induced by 1,25(OH)2D3; and 3) 12-O-tetradecanoyl phorbol 13-acetate and 1,2-dioctanoyl-sn-glycerol, known activators of protein kinase C, also rapidly stimulated rat colonic particulate guanylate cyclase activity. Taken together, these results demonstrate that 1,25(OH)2D3 rapidly stimulates together, these results demonstrate that 1,25(OH)2D3 rapidly stimulates rat colonic particulate guanylate cyclase, at least in part, via a protein kinase C-dependent mechanism.