Median lethal concentrations for 4-hour exposure of rodents to diborane were determined to facilitate comparison of the acute toxicities of various boron compounds. These LC50 values were 80 ppm for 5-month-old Chemical Warfare Laboratories Colony rats, 40 ppm for 2-month-old Edgewood Breeding Farms rats, and 29 ppm for Carworth Farms mice. Evidence from an additional experiment with the two strains of rats suggests that the difference in LC50 values for the rats results in greater measure from the age or weight difference than from the strain difference. Flies were also exposed to diborane, and results from these exposures show that flies are not useful as diborane detectors.