The absorption and excretion of dimethyl sulfoxide (DMSO) were studied in Rhesus monkeys (Macaca mulatta) given daily oral doses of 3 gms DMSO/kg B.W. for 14 days. DMSO and its major metabolite, dimethyl sulfone (DMSO2), were measured in serum, urine and feces by gas-liquid chromatography. DMSO was absorbed rapidly, reached a steady state blood level after 1 day and then was cleared from blood within 72 hrs after ending treatment. Serum DMSO declined in a linear fashion on semilogarithmic coordinates as described by second order kinetics. It had a half-life of 16 hrs. DMSO2 appeared in blood within 2 hrs and reached a steady state concentration after 4 days of treatment. DMSO2 was cleared from blood about 120 hrs after DMSO administration was stopped. Its half-life in blood was calculated to be 38 hrs. Urinary excretion of unmetabolized DMSO and DMSO2 accounted for about 60% and 16%, respectively, of the total ingested dose. Neither DMSO nor DMSO2 was detected in fecal samples. However, when added to fecal samples, DMSO was degraded rapidly. Although dimethyl sulfide (DMS) was not measured, some DMSO was metabolized to this compound because of the particular sweetness of breath of the monkeys. We conclude that the absorption of DMSO by monkeys is similar to that for humans, but that its conversion to DMSO2 and urinary elimination are more rapid in monkeys.