Technegas and Pertechnegas are radioactive aerosols produced in a commercial generator and used for lung scintigraphy. The aerosols are produced by first evaporating to dryness standard technetium-99m generator eluate (99mTcO4 in normal saline) in a graphite crucible (the simmer stage) and then heating this to 2500 degrees C (the "burn" stage). The aim of this work was to measure the particle size distributions of these agents and relate this to regional lung deposition. Factors that may vary during production of the aerosol in routine use were investigated to determine whether they influenced the particle size. Activity size distributions were measured using a serial wire-screen diffusion battery. The Technegas size distribution was approximately log-normal with an activity median diameter of 158 nm and a geometric standard deviation of 1.5. The results for Pertechnegas were similar. The median size increased with the number of simmers and with the time from generation. The increase in size with the number of simmers is thought to be due to the increased salt content in the crucible prior to the "burn". The predicted lung deposition is 37% in the alveolar region and 5% in the bronchial region. Significant changes in deposition are not predicted over the range of particle sizes measured.