A circuit is described which is designed to measure the time elapsed between a fast‐rising signal from a nuclear detector and the next pulse from a steady source of pulses which are evenly spaced in time and occur at an rf rate. This circuit has been found useful for measuring time intervals in the millimicrosecond range as part of a system of fast‐neutron spectrometry which utilizes a pulsed source of neutrons. The conversion of time to pulse height is linear for time intervals from 10 mμsec to a time which is 20 mμsec shorter than the interval between the rf pulses. The resolution obtainable depends on the characteristics of the detector. The precision of time measurement is about 1 mμsec.