Peritonitis is an inflammation of the peritoneal cavity and is one of the main causes of animal deaths. It has been reported that many diseases such as peritonitis cause electrolyte imbalance in the body. The present study has been conducted to evaluate the serum electrolyte concentration in cattle with peritonitis. In order to perform this study, 45 cattle with peritonitis were selected in the Karaj area, and 20 healthy cattle were used as the control group. After diagnosis of peritonitis in the infected cattle, 10-ml blood samples were taken from the jugular vein, the concentrations of calcium, phosphorus, magnesium, and chloride were estimated using the spectrophotometric method, and sodium and potassium concentrations were assessed by a flame photometer device. The results showed that the concentrations of calcium, magnesium, sodium, potassium, and chloride in cattle affected with peritonitis were reduced compared with the control group, but the differences were not statistically significant. The concentration of phosphorus in the peritonitis-infected cattle was significantly higher than in the healthy cattle. On the basis of the results of the present study, it can be concluded that inflammation of the peritoneal cavity in cattle causes blood electrolyte deterioration, and more attention needs to be focused on this factor in the treatment of infected animals.