We examined hemolymph O_2 partial pressure, pH, total CO_2 content, CO_2 partial pressure and bicarbonate concentration in order to evaluate the acid–base balance of the Pacific oyster Crassostrea gigas after pretreatment of the adductor muscle by cannulation. The hemolymph O_2 and acid–base properties changed just after surgery. The temporary and significantly fluctuation of hemolymph properties disappeared at 1h after surgery in this study, and the O_2 and acid–base status was stable afterwards in normoxic conditions. The results in this study showed the possibility that sampling with a cannula can collect hemolymph as required. This sampling may be useful, when respiratory and endocrine function must be monitored in minimally disturbed animals without the effects of handling.