A microprocessor-based system devoted to ambulatory monitoring of heart-rate variability (HRV) is described. Mainly it measures R-R intervals after QRS detection and provides a real-time quantification of the HRV. The concept of derived successions and a suitable set of graphical summaries are used. The hardware is based on CMOS technology; the mobile part of the system is battery operated. The results are printed-out by the fixed part of the system, they can also be fed into a general-purpose computer. Other applications of the system are suggested.