Various organophosphorus compounds are widely used in the world. Each organophosphorus compound has some differences in toxic symptoms, inhibition of ChE (cholinesterase) and the efficacy of PAM (2-pyridine aldoxime methiodide). This paper reports on the effects of three organophosphorus compounds (1/2 LD50, p. o.) and PAM on ChE activity in rat tissues including the relation to toxic signs. The results were as follows:1. Toxic signs appeared in the respiratory function of rats immediately after administration of Dichlorvos, and these signs disappeared rapidly. But in the case of rats administered Fenitrothion or Pyridaphenthion, serious toxic signs such as salivation, dyspnoea, lacrimation and tremors caused decreased body weight and rise of hematcrit value, and these signs continued for several days.2. ChE activity in rat tissues administered organophosphorus compounds spontaneously recovered further after the toxic signs disappeared. But there was a clear relation between the inhibition levels of tissue ChE and the appearance of toxic signs in rats administered three organophosphorus compounds. With special reference to the correlation between salivation and the inhibition level of ChE activity in salivary glands, serious salivary secretion was observed in rats where ChE activity in the glands was inhibited severely by administration of Fenitrothion or Pyridaphenthion. It was clear that salivary secretion was induced by organophosphorus compounds when the activity of ChE in salivary glands was inhibited by more than 50% of normal activity.3. The half-time (r1/2) for spontaneous recovery of tissues' ChE activity in rats following poisoning with organophosphorus compounds was different by tissues. But the r1/2 for recovery of serum and liver ChE activity was the same as the approximate 20h for three organophosphorus compounds, and those were shorter than other tissues' ChE. The r1/2 for recovery of erythrocytes ChE was longest in tissues, particularly those of rats treated with Fenitrothion was as long as 410h.4. As expected, atropine was unable entirely to reactivate tissue ChE activity inhibited by three organophosphorus compounds. The antidote PAM was capable of reactivating ChE activity in only serum and erythrocytes of rats administered Pyridaphenthion, but it couldn't reactivate ChE activity in brain, liver, kidneys, lung and salivary glands for pyridaphenthion, nor ChE activity in any tissues for Dichlorvos and Fenitrothion.Therefore, it was clear that each ChE in rat tissues has many differences in the speed of spontaneous recovery and the reactivating effect of PAM following poisoning with organophosphorus compounds. The qualitative differences of ChE by each tissue will be made clear in the future by a study on ChE isoenzyme of rat tissues.