A group of 7 healthy males was studied after maximal exercise and during and after prolonged exercise of two types (approximately at 40 and 67% of their VO2 max). Hematocrit, plasma proteins concentration, and hemoglobin were followed. Relative changes of plasma volume and total content of plasma protein were calculated from hematocrit changes. The mean corpuscular hemoglobin concentration was obtained by dividing hemoglobin by hematocrit. After maximal exercise, hemoconcentration with concomitant decrease of plasma volume (-13.0%) was found, with a corresponding increase in protein concentration (+12.9%) and without any protein content changes. Later normalisation at the 30th min of the recovery phase was shown. During prolonged submaximal exercise (67% of VO2 max) the changes in hemoconcentration, plasma volume (-7.1%), and plasma protein concentration (+6.9%) reached the highest changes in the first 15 min of exercise, and no decrease in protein content was observed. After them the spontanious tendency to the normalisation was found. No changes were registered in prolonged mild exercise (40% of VO2 max). The ratio of hemoglobin to hematocrit remained unchanged during and after all types of exercise.