In order to assess the effects of environmental stimulation on the perceptual thresholds of high active and low active mentally retarded persons, 36 male and female residents from a state institution (mean IQ 56 and 60 for high and low groups, respectively) were observed under three conditions of environmental stimulation: no noise, white noise, and cafeteria noise. Each subject's activity level was recorded for exactly five minutes, on each occasion, by means of an ultrasonic motion detector, and was compiled by an electromagnetic counter. Results showed that the high- and low-active mentally retarded persons did not differ in perceptual threshold measures under the restricted auditory conditions. There were no significant differences between all of the possible comparisons of level of activity with the other dimensions of the study or with any of the associated interactions. An extensive list of references and a detailed review of the literature accompany the study. (RD)