Abstract: A time interval recorder stores the magnitude of time intervals between events, e.g. heartbeats, over a period. The data is stored as a sequence of pairs of numbers representing respectively the magnitude of a time interval and the number of successive occurrences of the same time interval. The recorder comprises a digital timer, a comparator, a counter and a digital store. The timer provides a digital indication of each successive time interval which is compared in the comparator with the immediately previous digital indication. If there is equality then the counter is incremented. If there is non-equality then the count is stored in the store together with the digital value of the previous time interval and the counter is then zeroed. The store may also store digital indicators of elapsed time in the same sequence of stored numbers.