The relationship between gastric acidity and the bioavailability of two kinds of sustained-release indomethacin (IM) formulations was investigated in gastric acidity-controlled beagle dogs, and compared with that of rapid-release IM formulations.All test dosage forms were more rapidly dissolved in simulated intestinal fluid than in simulated gastric fluid.Gastric acidity did not affect the bioavailability of IM from the rapid-release formulation. However, the bioavailability of IM from the two kinds of sustained-release formulations were markedly influenced by gastric acidity. The rates of IM bioavailability from both of the sustained-release formulations were faster under low acidity conditions than under high acidity conditions (p<0.01). In addition, Tmax and mean residence time (MRT) were approximately the same for the rapid-release and sustained-release formulations under low acidity conditions.These results suggest that the IM sustained-release formulations showed a rapid-release profile under low acidity conditions.