Excystation of coccidian oocysts is a diphasic process where CO2 and suitable reducing agents cause changes in oocyst wall permeability (phase I) allowing the entrance of trypsin and bile which activate the sporozoites (phase II). This project was an indirect study of the mechanism of CO2 action by the substitution of NO, NO2, N2O, H2S, SO2 , CH4, NH3 and 8M urea in place of CO2. Changes in oocyst wall permeability of Eimeria stiedae, E. bovis and E. tenella were determined by incubation with the reagents and cysteine HCl followed by treatment with trypsin and bile to initiate activation of sporozoites, staining oocyst inner structures with methylene blue, and removal of outer and inner oocyst walls with sodium hypochlorite. The gases CH4 , NO2, N2O were negative for all 3 tests as were SO2, NH3 and 8M urea which in addition were toxic to the oocysts. Both H2S and NO were capable of mimicing the action of CO2 and are related chemically to the reducing agent, and hence tend to underscore its importance in excystation. It now appears that the role of CO2 is that of an allosteric effector enhancing the action of the reducing agent.