When segments excised from the etiolated hypocotyls of Gossypium hirsutum are pretreated in buffer, the subsequent uptake of radioactive 2,4-dichlorophenoxyacetic acid (2,4-D-1-14C) is depressed and the net loss of radioactivity which normally follows a phase of positive uptake by freshly excised segments does not take place. Uptake by fresh segments, in contrast with uptake after pretreatment, has a high Q10 and is markedly depressed by both 2,4,5-trichlorophenoxyacetic acid (2,4,5-T) and 3-indolylacetic acid. On these grounds it is proposed that net loss results from the release of material accumulated by a specific mechanism which, with time, becomes inoperative. Additional experiments suggest that part of the 2,4-D taken up by stem segments of Triticum vulgare and Avena sativa is accumulated by a similar mechanism. For 1-cm segments, entry is most rapid through the cut ends, and the effects of pretreatment exert their maximal effects in the tissue near the ends. Therefore very short segments have been used to compare the courses of uptake of phenoxyacetic acid (POA) and its 2-, 4-, 2,6-, 2,4- and 2,4,5- chloro- derivatives. The patterns observed are similar to those previously reported for 1 -cm segments, although the differences between compounds are greater. The courses of uptake of 2,4-D and 2,4,5-T, both terminate in a phase when there is a net loss. POA and the 2-chloro-substituted acid (2-CPA) are both continuously accumulated. No net loss is found with either the 2,6- (2,6-D) or the 4- chloro (4-CPA) compounds but the rates of uptake progressively decrease to a low level. It is proposed that the processes which determine the pattern of uptake of chlorinated phenoxyacetic acids include two types of accumulation. With Type I accumulation the mechanisms involved rapidly become disorganized after tissues are excised from the plant. Type 2 accumulation, on the other hand, is stable. The available data indicate that Type I accumulation is peculiar to compounds with marked auxin-like properties.