20 adult female macaques (Macaca fascicularis) were given oral doses of L-selenomethionine (L-SeMet) equivalent to 0, 25, 150, 300 and 600 micrograms selenium (Se)/kg body weight, and plasma, erythrocyte, hair, faecal and urine Se concentrations were determined. The macaques were scheduled for 30 daily oral doses of L-SeMet, but systemic toxicity necessitated dose reduction in several animals; two macaques given 600 micrograms Se/kg body weight/day for 10-15 days died, and the concentration of Se in their tissues was determined and compared with Se concentrations in tissues collected from one untreated animal. Circulating and urinary Se concentrations in control macaques were within the normal human ranges. Plasma, erythrocyte, hair and urinary Se concentrations were generally dependent on the dose of L-SeMet administered. Plasma Se reflected more immediately exposure to L-SeMet, whereas erythrocyte Se concentrations increased and decreased more slowly. In some cases, erythrocyte Se was still increasing or showed a plateau after L-SeMet treatment was discontinued. Plasma Se concentrations of 6.7-7.3 ppm were observed in the two animals that died due to acute toxicity to L-SeMet. Neither plasma nor erythrocyte GPx activity was influenced by a single L-SeMet dose, but an increase in erythrocyte GPx activity occurred with continuous exposure. Total tissue Se increased 13-28-fold in macaques given 600 micrograms Se/kg body weight/day for 10-15 days, with the liver and kidneys containing the the highest Se concentrations.