For the study of blood pressure, the use of automatic fully ambulatory recording systems greatly facilitates data collection and, in combination with proper software, the interpretation of results. Hardware for automatic long-term measurement is here wedded to a set of software modules to assess the circadian variability of human blood pressure. To evaluate changes in blood pressure within a day, the authors analyzed 150 series of systolic and diastolic blood pressures and heart rates automatically monitored every 30 minutes for at least 48 hours. Subjects were clinically healthy men and women 19-23 years of age. Individual circadian-rhythm parameters computed for each time series by least-squares rhythmometry were used to assess rhythm characteristics of all subjects and for each gender separately. Those individual parameters were also used for the computation of a so-called paradesm, a (here 90%) time-specified prediction region for the amplitude-acrophase pair. The limits thus obtained for the circadian amplitude (an index of within-day variability) of blood pressure were exceeded in cases of amplitude-hypertension. Additionally, the original data were used to compute time-specified prediction limits for blood pressure and heart rate in clinical health. Such reference limits may serve for an objective and positive definition of health, for the screening and diagnosis of disease, and for gauging a subject's response to treatment. The combined hardware-software method for the establishment of time-qualified reference limits presented allows the assessment of the extent and timing of rhythmic change and can be used for the interpretation of single values obtained at specified times.