Improved hydrocracking process

A process for hydrocracking hydrocarbon fractions boiling above 300 DEG F. by contact with a hydrocracking catalyst at a temperature of 500 DEG to 800 DEG F. for obtaining a high octane product of relatively constant quality throughout the reaction period comprises contacting said fraction with a hydrocracking catalyst comprising a platinum group metal on a crystalline aluminosilicate zeolite having an effective pore opening of 6 to 15 Angstroms which has been base exchanged with a cation to reduce its Na2O content to less than 10%, maintaining a high concentration of a catalyst poison during the initial stages of hydrocracking and reducing the concentration of the catalyst poison during the subsequent aging of catalyst while maintaining substantially constant the feed rate, conversion level and reaction temperature. The catalyst poison may be a nitrogenous compound or carbon monoxide. The process permits operation at a constant high temperature thus giving a high-octane product but avoids the excessive production of gases characteristic of the use of high temperature with active catalysts.