Gammalin-20 is a widely used organochlorine pesticide in veterinary and human medicine for the treatment of ectoparasite and pediculosis. This study was aimed at determining the effects of drinking Gammalin-20 contaminated water on some biochemical parameters in male Rattus norvegicus rats. Each of the ten rats in the experimental group drank approximately 2.5 Liters-3.0 Liters of 0.01% Gammalin-20 contaminated water within the period of four weeks which this study lasted while each of the ten rats in the control group drank approximately 10 Liters- 12 Liters of distilled water within the same period. Five milliliters blood specimens were collected from each experimental and control rats into lithium heparin anti-coagulated bottles. The blood specimens were spun and plasma samples used for the quantitative measurement of alanine aminotransferse, aspartate aminotransferase, urea, creatinine and C-reactive protein using a spectrophotometer. The results revealed statistically significant elevations (p<0.05) in the mean values of all the biochemical parameters measured in the experimental rats as compared to that of the control. In conclusion, drinking of 0.01% Gammalin-20 contaminated water for a period of four weeks may lead to hepato-renal and inflammatory disorders in Rattus norvegicus rats. It is therefore recommended that the use of Gammalin-20 in veterinary, killing of fish, human medicine etc should be well guided and properly regulated so as to avoid the likelihood of these adverse effects on humans