Enough evidence (3) has accumulated to show that ethylene is a fruit ripening hormone. Ethylene evolution in fruit tissues accompanies processes of maturation and aging. Although work on the metabolic changes caused by artificial doses of ethylene has appeared (2,4,7), the real nature of ethylene reactions and its relation to metabolites in inducing fruit ripening are still not clearly understood. With the multitude of changes that occur in ripening fruit, no one change has been shown to be initiated directly by ethylene. In mangoes not much is known about the exact role of ethylene action. Mangoes, which were thought to be nonclimacteric I(1), have been found to have a normal pattern of ethylene evolution which coincides with their respiratory peak 1(2). We have observed that Alfanso mangoes do produce ethylene (6) using gas chromatographic method (5). Together with the ethylene evolution and respiratory climacteric in mangoes, the catalase and peroxidase activities were found to increase considerably, due to the disappearance of a heat-labile and nondialyzable inhibitor of these enzymes. Using mango slices further investigations were carried out to study the effect of ethylene on these enzyme activities and also on their endogenous inhibitor. Alfanso mangoes (Mangifera indica) were used in these investigations. The mangoes used in the experiments were surface sterilized, and then cut into uniform slices. Slices of unripe (preclimacteric) tissue (2 g fr wt) were placed in 5 liter containers. Ethylene mixtures of 10 ppm and 50 ppm were introduced into these chambers using the evacuation method (9). The chambers were washed and rinsed with alcohol before use. A beaker containing solid NaOH was kept in the center of the container to absorb CO2 evolved. Unripe tissue slices inculbated in a similar container in the absence of ethylene served as controls. The containers were sealed and incubated at 25°. At various time intervals (17 hr, 24 hr, 45 hr, and 70 hr) the slices were removed for analysis. Containers were unsealed at these times and exposed to air for 1 or 2 hr, and then once again the required amount of ethylene was introduced. The treated slices and their controls were, after removal, cooled to 0° and immediately extracted for the determination of enzyme activities and the inhibitor concentration.