Naturally occurring plant growth regulators gibberellic acid (GA), indole‐3‐acetic acid (IAA), abscisic acid (ABA), ethylene and other growth regulating compounds such as 5‐methyl‐7‐chloro‐4‐ethoxycarbonylmethoxy‐2,1,3‐benzothiadiazole (TH) and 2,4 dichlorophenoxyacetic acid (2,4‐D), had no effect on the partition behavior of a piperidine based spin label in liposomes composed of pure or mixed phospholipids or a phospholipid‐sterol mixture. Although no effect on fluidity was observed, TH significantly increased the initial rate of swelling of soybean lecithin‐sitosterol liposomes in isotonic glycerol. IAA and ethylene did not influence this rate but ABA, GA and 2,4‐D inhibited the initial rate of swelling. Lipid composition of liposomes determined the extent and direction of the effects on swelling rates. The observed swelling behavior was, therefore, not related to fluidity of the bulk membrane lipids but was due, instead, to modification of the access of glycerol to the phospholipid bilayer surface or, alternatively, to the creation of polar channels into the liposomes.