The ambulatory continuous arterial pressure monitoring permits a more exact definition of the degree of arterial hypertension. A ACMPA was performed in 86 subjects, 64 hypertensive and 22 normotensive controls during a daytime period of 16 hour with a noninvasive device. The results were compared to those obtained in the four four-hour subperiods. The correlation of each subperiod with the global studied period was: r = 0.915; subperiod 16-19 hours, r = 0.933; subperiod 20-23 hours, r = 0.924. It was observed that in any four-hour subperiod was quite similar to the mean value of the whole period. We conclude that in those cases in which it is not possible to perform a 24 hour ACMAP or during the whole daytime, a short four-hour monitoring is valid.