External application of abscisic acid (ABA) induces a relatively high rate of xylem exudation in excised roots ofLupinus luteusL. cv. Weiko III. The response is relatively slow with a lag period of ca 1 h. It is also slowly, but reversibly, abolished by application of 3.6 or 36 μMcycloheximide (CHX). Contrary to expectation, K+is not a significant factor in maintaining flow rates in lupin roots as no response was measured after adding K+to root systems, from which K+had been withheld for periods ranging from 3 h to several days. In fact, excised roots obtained from seedlings raised in the absence of K+failed to respond to added K+. Total depletion of K+is difficult to achieve, because of initial seed reserves, and prolonged exudation in lupins can be maintained utilising only a small proportion of the K+originally present in the root tissue. Nevertheless, the data cast doubt on the general applicability of the concept that volume flow is maintained by an osmotic gradient with K+as the principal mineral ion