Leaf segments of tomato plants (Lycopersicon esculentum Mill.) of a normal strain and of two nonripening mutants rin and nor were aged in darkness. Respiration in leaf segments of all strains followed a climacteric-like pattern which was accompanied by a similar pattern of ethylene production. l-Methionine-U-14C vacuum-infiltrated into leaf segments at the beginning of the climacteric-like rise in respiration was metabolized to ethylene and CO2 during the subsequent 48 hours to about the same extent in all strains. Pericarp disks of immature fruits of all strains also metabolized l-methionine-U-14C to ethylene and CO2 to about the same extent during the first 48 hours following cutting and vacuum infiltration. Conversion of methionine to ethylene in disks was much more efficient than in aging leaf segments. The apparent capacity for increased production of ethylene in aging leaf segments and in response to wounding in pericap disks of rin and nor is contrasted with the absence of a respiratory climacteric and an associated large increase in ethylene production during natural aging of intact fruits of these two strains.