A procedure is described which permits the estimation of the relative activity of phenylalanine ammonia-lyase (E.C. 4.3.1.5.) in intact plant cells, exemplified by buckwheat hypocotyls. Hypocotyl segments are incubated at pH 5.5 with L-[3-(3)H]phenylalanine. N(3)HH2, which is liberated from phenylalanine by the action of phenylalanine ammonia-lyase, equilibrates with tissue water to yield (3)HOH, which is recovered by sublimation. Participation of phenylalanine transaminase in the reactions leading to (3)HOH formation is excluded, and it is conclusively shown that (3)HOH is formed intracellularly and not by enzymatic activity leaking out of wounded tissue.