In the electrically stimulated guinea-pig ileum, which was incubated in the modified Krebs-Henseleit bicarbonate buffer solution containing various concentrations of electrolytes at 4˚C for 24 hours, the effect of naloxone on the inhibitory action of morphine was investigated. Incubation potentiated the inhibitory action of morphine. In the incubated preperation, the inhibitory action of morphine was potentiated in the Na+ 75mM, and K+ 2.9mM groups, while that action of morphine was reduced in the Ca++ 3.6mM, Mg++ free and Mn++ 0.2mM groups. Naloxone in incubation media potentiated in the inhibitory action of morphine. In the preparations which were incubated in various concentrations of electrolytes plus naloxone, the action of morphine was reduced in Na+ 75mM, K+ 2.9mM, and Ca++ 3.6mM groups, while that action of morphine was potentiated in Mg++free and Mn++ 0.2mM groups. Naloxone antagonised those actions of morphine. However, pA2 values for naloxone (index for affinity for antagonist) was not changed. Thus changes in the inhitory action of morphine caused by incubation are probably not the result of changes in the affinity of receptor, but due to the alterations in the events which precede or follow the receptor binding by incubations.