Analyses of gas in the chamber of the accessory organ of Anabas scandens were made by keeping the fish forcibly under running or stagnant water for various times upto 15 minutes immediately after it took a fresh gulp of air. The percentage of O2 and CO2 decreased variously as time of submersion increased. The O2 depletion was more rapid and the CO2 depletion less so in stagnant than in running water. The accessory organ is as good an outlet for CO2 in this species as are the gills and skin in other species. The time of triggering for the fish to surface for a fresh gulp of air depends on a definite O2-CO2 relationship in the accessory chamber.