Russell, R. W., Watson, R. H. J., and Frankenhaeuser, M. Effects of chronic reductions in brain cholinesterase activity on acquisition and extinction of a conditioned avoidance response.Scand. J. Psychol., 1961,2, 21–29.—Significant effects of five different levels of reduction in brain ChE activity in the white rat were found during the extinction, but not during the acquisition, of a conditioned avoidance response. The reductions were produced and chronically maintained throughout the experiment by administration of an organo‐phosphorous anticholinesterase. (1) Reduced brain ChE activity was associated with differential effects on the behavior, i.e., speed of conditioning was not altered significantly, whereas speed of extinction was so affected. (2) There appears to be a ‘critical level’ between 60 and 65 per cent reduction of normal ChE activity below which extinction was significantly