The NL254 Ratemeter calculates, using digital circuitry, the ‘instantaneous frequency’ of each interval in trains of events, uses a 3 digit display to show the rate in pulses per second or pulses per minute and produces an analogue voltage which is proportional to this rate. It features an adjustable threshold so that additional modules for detecting peaks (such as theNL201 Spike Trigger) should not be necessary. The NL254 Ratemeter has four ranges which allow it to be configured for low frequency events such as heart or respiration rate as well as higher frequency signals such as neuronal action potential firing. The NL254 combines and replaces the functions of the NL253 and NL256 which are no longer available.