[en] Leaf discs and detached leaves exposed to L-cysteine emitted a volatile sulfur compound which was proven by gas chromatography to be H₂S. This phenomenon was demonstrated in all nine species tested (Cucumis sativus, Cucurbita pepo, Nicotiana tabacum, Coleus blumei, Beta vulgaris, Phaseolus vulgaris, Medicago sativa, Hordeum vulgare, and Gossypium hirsutum). The emission of volatile sulfur by cucumber leaves occurred in the dark at a similar rate to that in the light. The emission of leaf discs reached the maximal rate, more than 40 picomoles per minute per square centimeter, 2 to 4 hours after starting exposure to L-cysteine; then it decreased. In the case of detached leaves, the maximum occurred 5 to 10 h after starting exposure. The average emission rate of H₂S during the first 4 hours from leaf discs of cucurbits in response to 10 millimolar L-cysteine, was usually more than 40 picomoles per minute per square centimeter, i.e. 0.24 micromoles per hour per square decimeter. Leaf discs exposed to 1 millimolar L-cysteine emitted only 2% as much as did the discs exposed to 10 millimolar L-cysteine. The emission from leaf discs and from detached leaves lasted for at least 5 and 15 hours, respectively. However, several hours after the maximal emission, injury of the leaves, manifested as chlorosis, was evident. H₂S emission was a specific consequence of exposure to L-cysteine; neither D-cysteine nor L-cysteine elicited H₂S emission. Aminooxyacetic acid, an inhibitor of pyridoxal phosphate dependent enzymes, inhibited the emission. In a cell free system from cucumber leaves, H₂S formation and its release occurred in response to L-cysteine. Feeding experiments with [³⁵S]t-cysteine showed that most of the sulfur in H₂S was derived from sulfur in the L-cysteine supplied