Prediction accuracy from models which adequately describe drug release from polymeric matrices depends on the availability of good estimations/measurements for drug diffusion coefficients. Lenses obtained from membranes with 2-hydroxyethyl methacrylate (HEMA) as the main component were used in this study. Two hydrogels were prepared by thermal copolymerization: HEMA with methacrylic acid (MAA, hydrophilic monomer) and HEMA with methyl methacrylate (MMA, hydrophobic monomer). Timolol maleate was incorporated into the polymeric matrices by soaking or by addition to the polymerization mixture (occlusion). An experimental methodology with continuous monitoring of drug concentration in the release medium was implemented, thereby avoiding time-consuming sequential manual sampling. Considering diffusional mass transport as the controlling mechanism, the value of timolol maleate diffusivity when impregnated by occlusion was (1.74 +/- 0.14) x 10(-12) m(2) s(-1) for lenses with MAA, and (1.66 +/- 0.14) X 10(-12) m(2) s(-1) for lenses containing MMA. When the drug was added by soaking, the value of diffusivity was (1.49 +/- 0.39) X 10(-12) m(2) s(-1) for lenses containing MAA and (1.11 +/- 0.26) x 10(-12) m(2) s(-1) for lenses with MMA. The hydrogel with greater hydrophilic characteristics presented higher values of drug diffusion coefficients than the more hydrophobic, an observation which was valid for both methods used to incorporate the drug. However, the increase is more significant (by about 34%) when timolol maleate is added to the polymeric matrix by soaking.