The kinetic parameters of in vivo ethylene metabolism by seedlings of Pisum sativum L. cv. Alaska have been determined. The oxidation of ethylene to CO2, (Ox) and the incorporation of ethylene into the tissue (TI) were both shown to display Michaelis-Menten kinetics (Km Ox = 0.9 × 10–6 M liquid phase, Vmax Ox = 2.4 × 10–10 moles g– dry mass h–1 Km TI = 1.6 × 10–6 M liquid phase, Vmax TI = 4.5 × 10–10 moles g–1 dry mass h–1). Propylene competitively inhibited both Ox (Ki = 7.0 × 10–6 M) and TI (Ki = 3.7 × 10–7 M). A system comparable to Ox was absent from imbibed cotyledons of Vicia faba L. cv. Aquadulce even at saturating concentrations of ethylene similar to those used in kinetic analysis on Pisum. Silver ions were shown to inhibit TI but promoted Ox, while carbon dioxide inhibited Ox but promoted TI. Kinetic data on both these effects are presented. Data on the effect of a range of concentrations of CO2 on TI and Ox are also presented.