Physiological dormancy of scarified seeds of Townsville stylo (Stylosanthes humilis H.B.K.) is broken by ethylene. When the biosynthesis of this gas was impaired by 2-aminoethoxyvinylglycine (AVG) plus Co2+, the response to ethylene at very low concentrations was appreciable in non-dormant seeds and nil in the dormant ones. Complete inhibition of germination of non-dormant seeds occurred only when they were treated with AVG plus Co2+ under an ethylene-free atmosphere, a condition in which no trace of the gas in the atmosphere of Erlenmeyer flasks could be detected. Injection of ethylene into that system triggered germination of both dormant and non-dormant seeds, demonstrating a requirement for the gas. Non-dormant seeds were at least 50-fold more sensitive to ethylene than the dormant ones. Perception of ethylene occurred within a very short time (at most 15 min), since exposure of both dormant and non-dormant seeds to the gas, at a steeply declining concentration, sufficed to cause substantial germination.