Diazocyclopentadiene (DACP) in the presence of fluorescent light delayed ripening of tomato fruits treated at the mature green (no visible red) stage . At 25 ยฐC, ripening was delayed 10 days if DACP [185 pl/l (gas)] was added as a single treatment and longer if DACP was added intermittently at 5-day intervals . The addition of 1000 yl/l ethylene following DACP and light treatment did not hasten ripening . Little ripening delay was noted for fruit + DACP held in darkness . Tomatoes covered with aluminum foil so as to exclude light but not light-activated DACP, showed ripening inhibition . Apparently, the light-activated product from DACP is stable long enough to diffuse into fruit held in darkness . After an initial inhibition, ethylene production was greatly increased in tomatoes treated with DACP . Tomatoes with or without DACP treatment were held either in air or 5% 0 2 /95% N2 for 12 days then treated with ethylene . Treatment with 5% 0 2 alone delayed ripening when compared to air alone, however, both groups reached 80% red color by 18 days . DACP treated fruit, whether held in air or 5% 0 2 , still were green after 18 days and only approached 80% red color after approximately 27 days . Thus, 5% oxygen did not appear to slow the reversal of DACP inhibition of ripening .