The time course of changes in breathing pattern in opioid-induced respiratory depression was characterised for two opioids. Intravenous morphine (0.039 mg.kg-1 bolus + 0.215 mg.kg-1.h-1 infusion) and oxycodone (0.05 mg.kg-1 bolus + 0.275 mg.kg-1.h-1 infusion) were administered to six healthy male volunteers for 2 h in a random, double-blind and cross-over fashion. Monitoring included pulse oximetry and noninvasive respiratory-inductive plethysmography for the measurement of breathing pattern. The total amounts of drugs given were 35.1 (0.0) mg [mean (SD)] morphine and 41.3 (8.0) mg oxycodone. Four of the six oxycodone infusions had to be stopped at 99 (14) min because of respiratory depression as judged by pulse oximetry. No morphine infusions were stopped. The first changes in breathing pattern were a decrease in respiratory rate and an increase in the contribution of the rib cage to tidal volume, while the compensatory increase in tidal volume became evident later. A decrease in minute ventilation and inspiratory duty cycle were also found.