Clinical blood tests on fish exposed to contaminants are used extensively both in laboratory and field studies. Seasonal variability is examined in some of the parameters commonly used in health assessments. Hematocrit, hemoglobin concentration and inorganic ion concentration were studied in perch, Perca fluviatilis, from a Baltic brackish water area. Seasonal cycles were demonstrated in hematocrit and in chloride, magnesium and potassium ion concentrations. Other parameters studied reacted irregularly or only slightly to the shift of seasons.