Ophthalmic timolol has been used for decades in the treatment of glaucoma and ocular hypertension, traditionally in aqueous 0.5% eye drops. Recently a timolol 0.1% hydrogel has been developed to improve systemic safety. The aim of the present study was to compare aqueous humor timolol concentrations after administration of 0.1% hydrogel and aqueous 0.5% timolol in patients scheduled for a cataract operation. The concentration in the aqueous humor was 210+/-175 ng/ml (mean+/-S.D.) 2h after administration of timolol 0.1% hydrogel and 538+/-304 ng/ml after aqueous 0.5% timolol. In the aqueous 0.5% timolol group more patients had unnecessarily high concentrations of timolol in the aqueous humor. beta(1)-receptors and beta(2)-receptors were practically 100% occupied after administration of both products. The hydrogel proved to be an excellent formulation in giving smaller inter-individual variation in penetration of timolol into the aqueous humor. Only a weak correlation was seen between corneal thickness and the aqueous humor concentration of timolol in the aqeuous 0.5% timolol group. In conclusion, in contrast to the conventional aqueous 0.5% timolol, 0.1% timolol hydrogel caused only slight inter-individual variation in timolol concentration in the aqueous humor.