In continuation of previous investigations on potentially muco-adhesive polymeric ophthalmic vehicles, a series of 24 solutions containing either cyclopentolate base ionically bound to different acidic polymers, or cyclopentolate hydrochloride in physiological solution or in different buffers, were tested for mydriatic activity in albino rabbits. The polymers were polygalacturonic acid, mesoglycan acid, carboxymethylchitin and carboxymethylamylose. The solutions containing cyclopentolate hydrochloride, two of which were commercial collyria, were tested for comparison purposes. Correlations were sought between the biological activity parameters of the preparations and some chemical and physicochemical parameters such as presence and type of buffer, viscosity, pH and buffer capacity. The investigation did not show any clear-cut effect of viscosity on the activity of the preparations, while the pH and the type of buffer appeared to influence the ocular bioavailability of the drug. In particular, the presence of borate ions appeared to increase the activity of cyclopentolate hydrochloride to a much greater extent than the pH. The polymeric preparations were more active with respect to solutions containing cyclopentolate hydrochloride, when the comparisons were made at similar pH values and when the latter solutions did not contain borate ions