Ocular and systemic absorption and antagonist activity of topical 1% cyclopentolate were studied in 11 elderly patients undergoing extracapsular cataract extraction, and in 8 healthy female volunteers. The patients received two 35 microl drops of cyclopentolate unilaterally and the healthy volunteers one 30 microl drop bilaterally to the lower conjunctival cul-de-sac of the eye. The drug concentrations were measured with radioreceptor assay and receptor occupancies with radiooccupancy assay using isolated rat brain muscarinic cholinoceptors. In the patient group, cyclopentolate concentrations in aqueous humour were approximately 3000 times higher than those in plasma. Muscarinic cholinoceptors were occupied totally (more than 99.9%) by aqueous humour and 3-18% by plasma taken at 55-125 min. after the drug application. In healthy volunteers peak plasma concentration of cyclopentolate, 2.06+/-0.86 (mean+/-S.D.) nM, occurred at 53 min., maximum receptor occupancy being 5.9+/-2.1%. The maximum pupillary dilatation occured at 30 min. after the drug application. At the same time the near point of vision was extended to more than 50 cm in all subjects. After topical application plasma receptor occupancy was not high enough to cause any significant changes in heart rate and in PQ time. None of the subjects experienced subjectively or objectively adverse effects to be attributed to cyclopentolate.