This paper considers a non-conservative pendulum-type non-linear oscillator which emulates some behavioural features of systems displaying adaptive response, or biological rhythms, under the influence of an environmental excitation (‘ Zeitgeber ’) of electromagnetic origin, for instance. Of special interest are : (a) The entraining or locking-in process of the oscillation under a sudden change Δω of the Zeitgeber frequency ; (b) the time interval within which locking-in is achieved, and (c) the evolution of the oscillation under constant excitation of varying amplitude