The influence of gamma radiation on the polarity of IAA translocation was investigated. Pieces of apple tree shoots, taken from 3year-old trees, were irradiated in a dark irradiation chamber BK-'"Co source, 10 kCi. Doses from 20 to 150 krad were given. Strips of bark were peeled off both irradiated and control shoots. The middle parts of the bark strips were placed on agar blocks loaded with IAA-1-'''C. The radioactivity of basal and acropetal segments was determined after about 20 h and results expressed as polarity quotient.It has been shown that irradiation with 20 krad (200 Gy) decreases the polarity quotient remarkably (polarity quotient of control equals 7; for 20 krad -2.9; 100 krad -1.6). The decrease in polarity results from inhibition of the basipetal translocation of IAA. Irradiated tissue shows a decrease in IAA uptake. Possible mechanisms of gamma irradiation effects on polar translocation of IAA are discussed.