In observations of three brain injured children (20, 32, and 24 months old), the authors found a strong association between reinforcement density and cooperative behavior during physical therapy sessions. The results suggested that the delivery of reinforcement per se has substantial effects on behavior. When large amounts of noncontingent positive reinforcement was provided each child, noncooperative behavior decreased very significantly. It appears that with some subjects, manipulation of reinforcement density may be a useful alternative or adjunct to manipulation of contingencies. (Author/DB)