Deposition in mouth and throat and the fraction of alveolarly deposited particles in the lung of 3.6- to 3.8-microns Teflon particles labeled with 99mTc were estimated in nine healthy subjects. The particles were inhaled in air or helium/oxygen mixture with a flow of 0.5 l/s by subjects with or without induced bronchoconstriction. The bronchoconstriction (two- to threefold increase in airway resistance) was induced by an aerosol of methacholine bromide. As the Reynolds number is three times lower for the helium/oxygen mixture than for air, and the sedimentation rate of the particles is about the same in both, a different regional deposition between particles suspended in air and helium/oxygen mixture should be due to turbulence. Deposition in mouth and throat did not differ significantly between air and the helium/oxygen mixture. The alveolarly deposited fraction tended to be larger for unconstricted airways and was significantly larger for constricted airways for inhalations in the helium/oxygen mixture compared to air. In real life, air pollutants and therapeutic aerosols may be inhaled with larger flow rates and broncho-constriction may be more pronounced in patients, so that deposition of particles due to turbulence can be important.