In antral mucous cells, acetylcholine (ACh, 1μM) activates Ca2+-regulated exocytosis, consisting of an initial peak that declines rapidly (initial phase) followed by a second slower decline (late phase) lasting during ACh stimulation. The initial phase is enhanced by cGMP (1). This enhancement was inhibited by a PKG inhibitor (Rp8BrPETcGMPS, 100 nM). However, Rp8BrPETcGMPS produced a delayed, but transient, increase in the exocytotic frequency during the late phase. On the other hand, cGMP is known to stimulate phosphodiesterase 2 (PDE2) which degrades cAMP (2). The aim of this study is to confirm that the delayed, but transient, increase induced by Rp8BrPETcGMPS was caused by inhibition of PDE2. Male guinea pigs were anaesthetized by pentobarbital-Na (70 mg/kg, ip.). Antral mucous cells were isolated by a collagenase digestion. The exocytotic events were observed by video-microscopy. The experiments were performed in accordance with the Guidelines of the Animal Research Committee of Osaka Medical College and the Guiding Principles for the Care and Use of Animals in the Field of Physiological Sciences (Physiological Society of Japan). ACh alone stimulated cGMP accumulation, which enhances the initial phase in ACh-stimulated antral mucous cells (1). Rp8BrPETcGMPS induced a delayed, but transient, increase in the frequency of the late phase, while it decreased the frequency of the initial phase. We examined the effects of a PKA inhibitor (PKI-amide) on the delayed and transient increase during the late phase induced by Rp8BrPETcGMPS in ACh-stimulated exocytotic events. A PKI-amide abolished the delayed and transient increase during the late phase induced by Rp8BrPETcGMPS. This suggests that Rp8BrPETcGMPS accumulates cAMP. Analyses of Western blot and immunohistochemistry demonstrated that PDE2A exists in antral mucous cells. A PDE2 inhibitor (BAY-60-7550) induced a delayed and transient increase in the frequency of the late phase in ACh-stimulated exocytosis similarly to Rp8BrPETcGMPS. The measurement of cAMP content in antral mucosae revealed that BAY-60-7550 accumulates cAMP similarly to Rp8BrPETcGMPS during ACh stimulation. These results suggest that, in antral mucous cells, Rp8BrPETcGMPS decreases the activity of the cGMP-dependent PDE2 by inhibiting PKG, leading to cAMP accumulation. Rp8BrPETcGMPS inhibited PDE2 in antral mucous cells, leading to cAMP accumulation by inhibiting cAMP degradation. This accumulation of cAMP induced the delayed, but transient, increase in the frequency of the Ca2+-regulated exocytotic events.