Abstract A response equivalent to ‘inspection time’ (IT) was developed using a reaction time task in which Ss were required to respond as quickly and accurately as possible to whichever one of two lights appeared on each of a number of trials. The appearance of the lights was followed on all trials by a backward mask. IT was estimated for each S as the shortest duration between the onsets of the light and the backward mask at which the S could respond accurately on virtually every trial. An estimate of ‘responding time’ was then made by setting the duration between the stimulus onset and the mask to each S's individual IT and gradually reducing the time between trials so that Ss had less and less time in which to respond correctly. Responding time was taken to be the shortest such interval at which the S could respond accurately on virtually every trial. This measure was found to be reliable although quantitatively different estimates were obtained using two different methods of presenting the stimulus trials. Responding time was longer for a group of lower-intelligence Ss than for a group of higher-intelligence Ss and the measure was also found to correlate with measured intelligence in the former group. However, it did not itself correlate significantly with IT suggesting that responding time and IT may be measuring different underlying processes.