The first step in the utilization of the N in urea appliedto the leaves of plants presumably is hydrolysis by the enzyme urease giving NH3 and CO2. The comparative rate constants of the activity-time curves of the enzymatic hydrolysis of carbon-14 labeled urea applied to the leaves of cucumber, bean, tomato, corn, celery, and potato were detd. by means of continuously monitoring and recording the accumulation of C14O2 in closed system. The initial reaction rates for cucumber, bean, tomato, and corn, the leaves of which were found relatively intolerant to urea, were typically of zero order. With celery and potato plants, having considerably more tolerance to foliar applied urea, the kinetics of the enzymatic hydrolysis of urea in the leaves were of first order. Equal molar concns. of sucrose used in soln. with urea and applied to the foliage of tomato plants significantly reduced the rate of urea hydrolysis. The plants most easily injured by urea sprays are apparently those which have the highest urease activity, and the initial kinetics of the reaction seem to be of zero order. Liability to urea injury is indicative of rapid hydrolysis and may denote rapid utilization. Hydrolysis as it occurs in the leaves of plants subsequent to foliar applns. of urea appears to be equivalent to utilization.