Abstract: There are several reports that indicate a linkage between exposures to power frequency (50-60 Hz) magnetic fields with abnormalities in the serum biochemical parameters. The present study was designed to understand whether power frequency magnetic fields could act as an environmental insult and invoke any biochemical changes in Guinea pigs. Male Guinea pigs were randomly divided into four groups each comprising of 36 animals. Group 1 as control was found sham-exposed animals. Groups 2 to 4 exposed to 50 Hz (0.0207 μT) in three different times (1, 2 and 4 h), respectively for 5 consecutive days. We found significant decreases in the levels of total protein, β- and y-globulins and in the activities of γ-glutamyltranspeptidase and malate dehydrogenase. The activity of aspartate aminotransferase decreased, but the activity of alanine aminotransferase was unchanged. Total lipid, cholesterol, triglycerides and pre-β-lipoproteins decreased, but α-lipoproteins, glucoseand cortisol levels increased. The most pronounced changes in the biochemical parameters studied were found in workers with the longest exposure to an electromagnetic field.