Rhododendron macrophyllum is a dominant species in.the shrub stage of secondary succession on burned and logged sites in the Tsuga heterophylla zone of the western Cascades of Oregon. A study was undertaken to determine if R. macrophyllum has the potential to inhibit the germination and growth of surrounding ~egetation through watersolub le toxins which . . are produced in its leaves and leached out of its litter by rainfall.Aqueous extracts of R. macrophyllum leaf litter significantly reduced germination and radicle growth in Btomus tectorum, ·Epilobitim ansustifolium, Picea sitChensis, ·pseudOtStiga·menzieSii, and·Tsu8a heterophylla. Osmotic pressure of the leaf extract solution was determined not to be a significant causative factor in the inhibition observed.Field studies showed four plant species to increase in frequency of presence with increasing distan~e from _1!. macrophyllum drip lines.Two species, both ericaceous, decreased in frequency of presence.Five species increased in density with increasing distance from R. macrophyllum drip lines, while only one species, which was ericaceous, decreased, It is postulated that these patte~s result fr~ the allelopathic influence of _1!. macrophyllum. Members of the Ericaceae are thought to either have a common response to environmental conditions or to be able to tolerate the toxins produced by members of thei~ family.