In this paper results of acute toxicity (oral and dermal), cumulative toxicity, subchronic toxicity, and conjunctiva irritation tests are reported. The mutagenic effects of fenthion, using SCE (in vivo) and UDS (in vitro) as indicators, were also tested. Histochemical changes in enzyme activities (including AChE, ATPase, and AKP) in tissues were observed. The results showed that the acute toxicity of fenthion belongs to highly toxic category. The highly cumulative effect of fenthion was also observed. The subchronic toxicity test, however, did not reveal any abnormal effects except the inhibition of ChE activity in plasma. The dose and ChE activity relationship equation was Y = 0.82x + 4.71. The SCE and UDS tests revealed no mutagenic potential. In histochemical experiments we found that AChE activity in tissues was markedly inhibited. AKP and ATPase activities at the cortex renis were increased in the treated rats.