The rate of hydration of lyophilised ophthalmic lamellae of graded density of 50 to 56 g/l in water or an aqueous solution of sodium chloride was estimated by means of a hydrophilic filtration membrane. With insignificant effect of density, the non-linear course of hydration is affected mainly by the diffusion mechanism of penetration of water into xerogel while preserving its shape in a time interval of to 20 minutes. In a period of 10 minutes the rate of hydration of 19.9 mm/min is decelerated to 15.3 mm/min in the presence of sodium chloride. This contributes to the elucidation of different rates of the release of active ingredients from ophthalmic lamellae of comparable density.

        Key words: xerogel – hydroxypropylmethylcellulose – hydration – kinetics