1. We have assessed the day-to-day reproducibility of intra-arterial blood pressure by monitoring 17 freely ambulant hypertensive patients for a period of 48 h. Eight had no change of therapeutic regimen throughout and nine took a single dose of a hypotensive agent before retiring on the second night. 2. Records were analysed to provide hourly mean values of heart rate, systolic and diastolic blood pressure. No significant differences between first and second day recordings were found except after the intervention in the second group. 3. Allowing subjects to follow their normal daily routine produces inevitable variation in their pattern of physical and other activity. However, by the use of these methods of recording and analysis, with pooled measurements from a small group of subjects, reproducibility is sufficiently good to permit the reliable assessment of therapeutic interventions.