Pyrethrins are obtained from the flowers of Chrysanthemum cinerarifolium. These are strong insecticides with low mammalian toxicity. The toxic effects of pyrethroid cypermethrin were studied using various biochemical parameters along with histopathological changes in a 30-day study in Wistar rats. The rats were divided into two groups. Rats of the test group were given sublethal doses of cypermethrin (14.5 mg/kg) by gavage once daily for 30 days and the control rats were given an equal volume of the vehicle. The animals were sacrificed on day 0, 10, 20 and 30 of the study. The results showed that cypermethrin caused a significant increase in the levels of serum aspartate transaminase, alanine transaminase, lactate dehydrogenase and plasma creatinine. It significantly lowered the levels of total proteins. The histopathological studies on various organs like liver, lungs, kidneys and heart were carried out. The changes in various biochemical parameters correlated well with the histopathological changes in various organs. In conclusion, the results of this study demonstrate that subacute oral administration of cypermethrin, even in low doses such as 1/10 LD 50 for 30 days induces toxic effects on different vital organs.