The mechanism of ethylene (C2H4) action on type I reactions (the loss of phytol, the loss of magnesium, and the modification of the side chains of an isocyclic ring of the chlorophyll (Chl) structure) during Chl catabolism in Citrus unshiu (C. unshiu) fruit was investigated by using inhibitors of protein synthesis. C2H4 treatment promoted the degreening through increased chlorophyllase (Chlase) activity of C. unshiu fruit peel held in the dark. Cycloheximide (CH), an inhibitor of cytoplasmic protein synthesis, inhibited the C2H4-enhanced degreening. Chloramphenicol (CP), an inhibitor of plastid protein synthesis, did not inhibit the effects of C2H4. On the other hand, C2H4 treatment did not significantly increase chlorophyllide (Chlide)-degrading enzyme (CDE) activity nor C132-carboxyl-pyropheophorbide (Pyrophed) forming enzyme (CPFE) activity of C. unshiu fruit peels. Likewise, CDE and CPFE activities in C2H4-treated C. unshiu fruit were unaffected by CH-and CP-treatments. A high positive correlation between the degree of greenness of C. unshiu fruit peel and Chlase activity was obtained : Y=1.368 X-16.038, (r=0.992), where X and Y are intensity of the green color of the peel and Chlase activity, respectively. However, correlations between the degree of greenness of the peel and 1) CDE activity (Y=0.024 X+98.030, r=0.460) and 2) CPFE activity (Y=0.015 X+103.581, r=0.424) are low. These results indicate that C2H4 appears to enhance the degreening of the peel through de novo synthesis of Chlase, and that Chlase is the "key" enzyme in the degreening process.