This paper deals with the behaviour of serum protein fractions in rats fed 22 resp. 52 weeks a 50% or 3% fat diet using Millipore electrophoresis. The total serum protein concentration was also estimated. In rats on the high-fat diet the albumin fraction significantly dropped, while elevated levels of alpha 1- and gamma-globulin fraction were found. Serum protein content was slightly elevated in rats fed the diet 22 weeks, but this elevation was only significant after 52 weeks feeding. Within the same diet group there were no significant differences observed.