The male rats were injected i.p. with a trace or a sublethal dose of [75Se]selenite (0.01 mg or 1.58 mg/kg Se of body weight, respectively). During seven days following the injection, the whole-body retention, organ distribution and excretion of 75Se were studied, along with 75Se distribution in blood and in blood fractions. Substantial dose-dependent differences in selenite metabolism were found: (i) The rate of 75Se excretion after the injection of the sublethal dose was observed to be substantially higher than with the trace dose - 86% of injected dose (% inj. d.) vs. 41% inj. d. during seven days. In addition to urinary excretion, the exhalation of 75Se took a considerable part in the former case. (ii) The sharp decrease of 75Se levels was a prevalent feature of the 75Se kinetics in most of the studied tissues after the injection of the sublethal dose. On the other hand, after the trace dose injection a considerable decrease of 75Se level was observed only in the liver. In the brain and particularly in the testis the 75Se level increased during seven days after this dose injection. The highest levels of radionuclide were found in liver after both doses. (iii) Over 80% of blood 75Se was contained in blood cells (RBC) at all time intervals studied following the sublethal dose injection. With the trace dose the blood 75Se was present predominantly in plasma. (iv) The distribution of 75Se in protein fractions of blood plasma and RBC-lysate was studied using gel filtration. The albumin fraction was found to be the main acceptor of 75Se in plasma 15 min after both trace and sublethal dose injection. However, in longer time intervals the 75Se distribution pattern in plasma proteins was affected by the dose applied. Most of 75Se present in RBC-lysate (68.5-91.2%) was detected in the haemoglobine fraction at all time intervals after the injection of the sublethal dose. On the other hand, with the trace dose this fraction contained only 7.5-28.5% of 75Se. Incorporation of a significant amount of 75Se to the GSH-Px (glutathione peroxidase) fraction of plasma and RBC-lysate was observed on day seven after the trace dose injection. GSH-Px represented 19% of plasma 75Se and 80% of 75Se present in RBC-lysate. Data on the molecular weight (M(r)) values of plasma and erythrocyte 75Se-GSH-Px were determined on the basis of elution volumes in the course of gel infiltration - 695,000 and 110,000, respectively. The noticeable difference in these values is discussed.