To investigate the resistance to extinction in mentally handicapped children under conditions of stimulus variation, three experiments involving mildly retarded children were conducted. Test procedures were altered somewhat in each case (the new learning task chosen for the second experiment being a reversal shift of experiment one), and the third experiment being a continuation of one which was an investigation of resistance to extinction as a function of the number of stimulus components common to acquisition and extinction. It was felt that the three experiments together provided a strong support for a stimulus variation explanation of extinction, as well as indicating that attempts to try alternative responses in extinction were retarded when the stimulus conditions of acquisition and extinction were similar. (CD)