The effects of growth retardants, 4-hydroxy-5-isopropyl-2-methylphenyltrimethylammonium chloride-1-piperidine carboxylate (AMO-1618 or AMO) and 2-chloroethyltrimethylammonium chloride (CCC), applied with and without gibberellic acid (GA3), on β-[(3)H]alanine uptake and leakage from pea (Pisum sativum L.) and betacyanin efflux from beetroot (Beta vulgaris L.) tissue were examined. Both compounds decreased the amount of β-[(3)H]alanine taken up into pea leaf discs, and increased the quantity of radioactive label that subsequently leaked out of this tissue. Efflux of betacyanin from slices of beetroot was also found to be promoted by treatment with CCC or AMO-1618. In no case were these effects reversed by application of GA3. It is concluded that the growth retardants may be altering tissue permeability by an interaction with the cell membranes, and this may account for some of the side effects of the retardants which cannot be explained on the basis of their inhibiting action on gibberellin synthesis.