The amt. of mercury vapor present in 50 dental operating rooms, dental laboratories and commercial dental laboratories was detd. by means of a General Electric Hg vapor detector of the instantaneous type. The vol. of Hg vapor was so low that interpolation on a calibration curve in ppm. of Hg was not practicable. Instead, meter readings were used, comparing them with the meter reading of a min. toxic amt. of Hg vapor, namely 50. The avg. finding for dental operating rooms was 3.2, with a range of from 0-12. Laboratories associated with dental offices averaged 4.1, and ranged from 2 to 6. Commercial laboratories averaged 8.6. The authors conclude that the amt. of Hg vapor in the avg. dental office or laboratory is not toxic.