The absorption, distribution, and excretion of 1,1- dimethylhydrazine (UDMH) have been studied in rats, rabbits, cats, dogs, and monkeys by use of radioactive tracer and colorimetric methods. The compound is rapidly absorbed and excreted regardless of route of administration, The vital organs of the body exhibit no preferential affinity for UDMH, and large amounts are excreted unchanged. Blood levels of UDMH could not be detected in animals receiving doses of less that 10 mg/kg. Urinary concentration is the most sensitive qualitative indicator of exposure to UDMH. If symptomatology, and blood and urine levels are carefully correlated, a diagnostic-prognostic estimate of exposure can be made.