A measuring system is described for the long-term continuous beat-to-beat measurement of mean aortic pressure and heart period (interval between beats) in the conscious unrestrained rabbit. Pressures and heart periods were collected in hourly histograms. The histograms were summed into day (0800-2000 h), night (2000-0800 h) and 24-hour histograms. Separate histograms were formed for resting and active periods. From each histogram the mean value, standard deviation, fifth percentile, 50th percentile and 95th percentile were calculated and stored on a floppy disc together with the histograms. Off-line, histograms and their descriptive parameters were plotted and sent to a HP1000 computer for bulk storage and further analysis. The measuring system is capable of handling up to eight animals simultaneously for an indefinite period. The design functioned properly in the long-term measurement (more than 4 weeks) of mean level and variability of arterial pressure and heart period. The measuring system is useful in the study of arterial blood pressure behaviour after sino-aortic denervation and in the evaluation of drug effects in conscious animals.