Seeds of ?pimenteira? (Xylopia sericea St. Hill.) show poor germination capacity even when mechanical or chemical treatments are applied. This experiment searched for the active principle responsible for this inhibition. The effect of phenolic extract on inhibition of lettuce seeds and hypocotyl growth was observed. Germinating lettuce seeds in either water or a solufion of GA3 (10-4) for three days, stopped the hypocotyl growth. Seeds in contact with phenolic extract failed to germinate even when GA3 was present in the medium. Presence of cafeic acid, ferulic acid, P-coumaric acid or umbelliferone was not detected.