1-methylcyclopropene (1-MCP) is an inhibitor of ethylene perception. In harvested climacteric fruit 1-MCP delays the rise in ethylene synthesis and respiration, resulting in delayed ripening and increased shelf life. 1-MCP does not always affect non-climacteric fruit. We quantified the rate of ethylene synthesis using steady-state flow-through gas exchange chambers and an automated thermal desoprtion gas chromatography system capable of quantifying 84 parts per trillion. This approach allowed whole plant ethylene synthesis to be continuously monitored over multiple days. 1-MCP application doubled the ethylene synthesis rate in tomato plants; treated plants returned to control levels after 4 days. In corn, there was a transient increase in synthesis (3 hours) when a high dose of 1-MCP was applied. 1-MCP had a minimal effect on ethylene synthesis in cotton plants.