The aim of this paper was to describe the time-course of the sedative effect of rectal chloral hydrate (75 mg/kg) in children undergoing CT scan or MRI. Twenty children (2.13 +/- 1.43 years old) were administered 75 mg/kg chloral hydrate rectally (chloralhydrat-rectiole rectal formulation, Dr Mann-Pharma Lab, Berlin, Germany), before a CT scan or an NMR imaging. Sedation was measured at specific times using a sedation score of 1-6. Patients were continuously monitored for respiratory and heart rate, systolic and diastolic blood pressures, and oxygen saturation. About 82.35 and 94.11% of the patients had a score of sedation > or = 3 within 15 and 30 min, respectively. The mean time to effective sedation (score > or = 3) was of 0.30 +/- 0.14 h (median time, 0.25 h). The mean duration of effective sedation (score > or = 3) was 1.29 +/- 1.05 h (median duration, 0.75 h). A total of 93.1% of the X-ray sections were obtained without artifact and sedation was considered by radiologists to be efficient for 83.3% of the procedures. This sedation procedure appeared efficient and safe during ambulatory CT scan and NMR imaging. The long-term effect of chloral hydrate, however, remains to be evaluated.