In a climacteric fruit such as apple, ethylene synthesis increases during ripening and cyanide is stoichiometrically produced with ethylene. β-cyanoalanine synthase (CAS) activity was found to increase in ripening apple from the preclimacteric to climacteric and to decrease in the post-climacteric stage. In the non-climacteric fruit pepper, ethylene production and CAS activity was 250 fold and 30 fold lower, respectively, than in preclimacteric apples. Treating peppers with ethylene induced a transient increase in ethylene production and a longer lasting rise in CAS activity. Both fruits evidenced cyanide insensitive respiration, apple at all stages of ripeness and pepper from the turning stage onward. The results are discussed with regard to mechanisms in fruit that protect tissue function against increasing amounts of cyanide.