We determined the effects of 1-methylcyclopropene (1-MCP) and ethylene on color change and CO2 and ethylene production in grapefruit. Treatment with 1-MCP at concentrations equal to or greater than 75 nL·L−1 inhibited ethylene-induced degreening, but increasing 1-MCP concentrations greater than 150 nL·L−1 did not cause additional inhibition of degreening. Although ethylene-induced degreening was inhibited by 1-MCP, the effect was transient. Treating grapefruit with 15 to 75 nL·L−1 1-MCP resulted in a slight suppression of CO2 production, whereas treatment with 150 or 300 nL·L−1 1-MCP resulted in rates of CO2 production significantly higher than nontreated fruit. 1-MCP treatment also caused a very pronounced increase in the rate of C2H4 production that was both dose- and time-dependent. The effects of 1-MCP on respiration and ethylene evolution were reduced if fruit was subsequently exposed to ethylene. Fruit treated with 1-MCP alone had the highest rates of CO2 production, fruit treated with ethylene after 1-MCP or ethylene alone had intermediate rates of CO2 production, and control fruit had the lowest rate of CO2 production. Rates of C2H4 evolution were ≈200 nL·kg−1·h−1 from control and C2H4-treated fruit compared with ≈10,000 nL·kg−1·h−1 from 1-MCP-treated fruit; fruit treated with ethylene after 1-MCP had ethylene production rates of ≈400 nL·kg−1·h−1. Our results lend further support for a regulatory role for ethylene in degreening of citrus and suggest that endogenous levels of ethylene regulate ethylene production.