The ventilatory effects of single i.v. doses of morphine 0.1 mg kg-1, pethidine 0.67 mg kg-1 and methadone 0.1 mg kg-1 were compared after ophthalmic surgery in an open, randomized study in 30 children aged 3-8 yr. Ventilatory changes after each drug had distinctive profiles, with appreciable individual variation. Acutely, the decrease in ventilatory frequency was greater with pethidine and methadone than with morphine. The acute decrease in oxygen saturation was greater with methadone and pethidine than with morphine. Methadone produced a greater and longer lasting increase in end-tidal carbon dioxide and greater decrease in end-tidal oxygen than morphine or pethidine. Changes in end-tidal carbon dioxide and oxygen concentrations and saturation were most transient after pethidine and of longest duration after methadone. No child developed apnoea or hypoventilation requiring assistance.