IN an earlier communication to Nature1 it was argued that the attempt to resolve the discrepancy in data on choice reaction time in relation to age in terms of the effective duration of signals2 must be regarded as inadequate. The observations were based on a small sample (N = 48) of data from a psycho-physiological study of the possible effects of ageing on professional (that is, airline, military and test) pilots. The present communication is a brief account of some of the further results obtained to date in a larger sample (N = 201).