MATERIALS AND METHODS Male Sprague-Dawley rats weighing 200 to 250 g were killed by decapitation. Bovine lung was obtained from a local abattoir. Liver, heart, lung, and cerebral cortex were rinsed in cold 0.25 M sucrose and homogenized in 0.25 M sucrose containing 10 mM TrislHCl buffer (pH 8.0), 1 mu EDTA, and 1 rnM dithiothreitol. Homogenates were centrifuged at 105,000 xg for 60 min. Supernatant fractions were used fresh or stored at-70”. In some experiments, soluble guanylate cyclase from rat liver and cerebral cortex was partially purified through the DEAE-cellulose chromatography step as described previously(3, 4, 13). Some supernatant fractions from rat heart and bovine lung were applied to Sephadex G-100 columns and eluted with 10 rnM Tris/HCl buffer (pH 7.6), 1 mM EDTA, and 1 mM dithiothreitol in order to separate guanylate cylcase from smaller