ABSTRACT.Stereotypy is one of the most common behaviours demonstrated by persons with mental handicaps. As such, it has generated a number of theories concerning its origin or maintenance. One of these theories, the homeostatic one, suggests that some persons engage in stereotypies and other motor behaviours in order to maintain a relatively constant level of responding. If this was indeed true, the fact would have important implications for treatment, i.e. those persons who varied rates of both stereotypy and other movements but retained an overall level of motoric responding would seem to be those for whom procedures like the differential reinforcement of incompatible behaviour would be highly appropriate. The purpose of this paper was to determine whether some people do engage in constant levels of responding, and it did so by collecting data on stereotypy and other motor movements of 12 persons with retardation. Collected throughout the sehool day for 5 consecutive days on micro‐computers, the data showed (1) that the stereotypic responding of four subjects was extraordinarily consistent, with the most deviant day's total being only 15 or 16% from the mean of the 5 days, (2) that other motor movements were inconsistent for all but one subject, but (3) that total movement (i.e. stereotypy plus other motor movements) was very high and was consistent for most subjects. The data were discussed in terms of assessing baselines for subjects for whom reinforcing adaptive motor movements would seem an appropriate means for reducing stereotypic respondin