The study describes long term ventilatory effects of 50 or 100 micrograms.kg-1 of morphine or 1.5 or 3.0 micrograms.kg-1 of buprenorphine when given in repeated intravenous (i.v.) doses, in a double blind fashion, to achieve equal levels of analgesia after thoracotomy. The patients were 56 children, six months to six years of age. Ventilatory rate (VR) was measured over the 24 h study period, and arterial carbon dioxide tension (PaCO2) was measured on arrival in the Paediatric Intensive Care Unit (PICU) and at 1, 6, 12 and 18 h. In the buprenorphine groups VRs progressively decreased during the first 2 h and remained significantly lower (P < 0.05) than in the morphine groups for 7 h. For the rest of the study period there were no differences. The PaCO2 values did not differ significantly at any point. For safety, prolonged observation of children is needed after intravenous administration of buprenorphine to ensure the ventilatory rate has stabilized.