The effects of intravenous morphine (10 mg/70 kg body weight) on ventilation in two groups of subjects, young (age 28-37 years) and old (age 65-82 years) were studied prior to elective surgery. There was no significant difference between the two groups in minute ventilation, ventilatory frequency and end-tidal carbon dioxide before morphine administration. In both groups there was a significant depression of ventilation and elevation of end tidal carbon dioxide tension 20 minutes after intravenous morphine. There was no significant difference between the two groups in the response of these variables to morphine. However, in the older group seven out of thirteen subjects showed frequent periods of apnoea or periodic breathing whereas three of the young subjects had these phenomena at much less frequent intervals. Respiratory depression after morphine is similar in old and young patients but old patients have more change in ventilatory control.