This paper is an extended form of a short report published previously (Eur. J. For. Path. 14, 1984b). Here, we describe the occurrence, the isolation, the chromatographic comparison, the biological activity, the measurement of concentration of p-hydroxy acetophenone (p-HAP) and its O-glucoside (Picein) by gas chromatography and their possible roles during stress phenomena. The gas chromatographic measurement of p-HAP and its O-glucoside (after enzymic hydroluysis) in the current year's shoots and branches showed that the ratio of p-HAP: p-HAP-O-glucoside is always greater than I in the affected trees and less than I in the healthy trees. The ratio of p-HAP:p-HAP-o-glucoside was shown to increase with increasing durations of water stress, indicating the involvement of p-HAP in stress phenomena. p-HAP was found to be phytotoxic to the 2-year-old saplings at a cone, of 500 ppm. p-HAP caused yellowing and browning of needles, needle-fall, inhibition of apical dominance (epicotyl, roots) and inhibition of bud-break in Norway spruce.