1 In a double-blind placebo controlled trial, zolpidem 10 mg, a new imidazopyridine hypnotic drug, was administered to 10 elderly female patients and placebo to 11, all recovering from hip and knee replacement surgery. Respiratory monitoring with an inductance plethysmograph and pulse oximeter showed that treatment over a 4 night period did not increase significantly the severity, frequency or duration of hypoxaemic episodes leading to SaO2 < 90% or < 85% when compared with placebo. Confidence intervals (corrected for baseline differences) for the median differences between the two groups on night 7, the fourth night of treatment, were from -1.85 to 0.480 and from -1.07 to 0 respectively for the frequency, and from -0.76 to 0.15 and -0.5 to 0 for the duration of the hypoxaemic episodes. The incidence of sleep related respiratory disturbances was not significantly increased compared with placebo on any night. 2 Respiratory monitoring using a simple inductance plethysmograph and pulse oximeter is acceptable to patients and staff. 3 The evaluation of all hypnotic and sedative drugs should include their effects on respiration during sleep.