Polymeric derivatives of plant growth regulators (phytoactive polymers) demonstrate their biological activity after releasing the low-molecular-weight bioactive substances by hydrolysis. In the present work the release rates for a series of polymeric esters of carboxyl-containing plant growth regulators of the auxin group (2,4-dichlorophenoxy acetic acid, 1-naphthylacetic acid, 2-naphthoxyacetic acid, and 2-naphthylthioacetic acid) were studied. The effective rate constants in experiments of alkaline hydrolysis decreased as the molecular mass of the polymer increased, the content of acid hydrophobic residues in polymer increased, and the acidity of the acid decreased. Data on the elucidation of stucture versus release-rate relationships in polymeric esters of plant growth regulators will further aid the preparation of novel delivery systems with optimal biological activity.