The effects of acute administration of diisopropyl fluorophosphate, Isofluorophate (DFP) 1.1 mg/kg SC on soluble brain acetylcholinesterase were studied in male Sprague-Dawley rats sacrificed at time intervals ranging from 3 hr to 25 days. Three main molecular forms of AChE were separated by polyacrylamide gel electrophoresis followed by enzymatic reaction with acetylthiocholine, staining and scanning densitometry for their quantitative evaluation. In some experiments the same three forms were separated by chromatography-gel filtration. In the brain of untreated animals the slowly-, medium- and fast-migrating forms accounted respectively for 64, 18 and 18% of the soluble AChE activity. At 3 hr after treatment with DFP, resulting in an 80% reduction of soluble AChE, the relative contribution of slowly-migrating forms to the residual enzymatic activity was decreased, while that of medium-forms was significantly increased. These changes became gradually more pronounced and reached their maximum at 4 days, when AChE had recovered to about 50% of control level. Subsequently, the distribution of the molecular forms showed a progressive return toward the control pattern. The partial recovery in the initial period after maximal enzyme depression was mainly due to an increase of medium-migrating forms. Thus these may be precursors of the biosynthesis of slowly-migrating forms and/or there may be functional specialization of different forms.