Amplified sensing offers the potential for high sensitivity; however, the vast majority of molecular strategies involve stoichiometric detection and signal transduction, including numerous recent examples of systems inspired by transition-metal-catalyzed reactions. Activation of latent precatalysts by a target analyte represents an attractive strategy for detecting low-concentration species. Analyte amplification represents another attractive approach, akin to PCR-based assays. Here we report an autocatalytic detection system based on the ethylene activation of Ru–I2 olefin metathesis precatalysts. Signal transduction is amplified by both catalytic ring closing metathesis of profluorescent substrates and ethylene propagation to activate additional units of catalyst. High sensitivity is observed as a result of this dual-mode amplified detection of ethylene. Detection of endogenous ethylene from fruit and oxidation-decomposition of polyunsaturated fatty acids via lipid peroxides is demonstrated.