The modification of the acetylcholinesterase (AChE) activity and of the acetylcholine (ACh) levels caused by diisopropylfluorophosphate (DFP) alone and in association with 300 mg/kg of diacetylmonoxime (DAM) have been studied in the olfactory brain, in the cortex and in the medulla-pons of the rat. DFP caused a reduction of the levels of ACh at 0.25 and 0.5 mg/kg. An increase of the levels of ACh was observed at 4.0 mg/kg of DFP only in the olfactory brain and in the cortex. A dose-dependent reduction of the AChE activity was observed after DFP. DAM reactivated the AChE activity to the levels of the controls after 0.25 and 0.5 mg/kg of DFP. In this experimental situation, the ACh levels returned to the control levels after 0.25 mg/kg of DFP and, in the medulla-pons, also after 0.5 mg/kg of DFP. DAM caused a partial recovery of the AChE activity but no modification of the ACh levels at 1.0, 2.0 and 4.0 mg/kg of DFP.