Weekly average concentrations of ammonia were measured in winter 1991-1992 in different rooms in houses and in a range of public buildings using passive diffusion tubes. Concentrations of NH3 ranging from 6 to 53 ppb were found in different rooms within a house. Concentrations in living rooms of 5 houses ranged from 7 to 63 ppb, with the largest values in rooms used by smokers. Concentrations of NH3 in public buildings were similar to those in houses, with concentrations in areas used for social activity greater than in work areas. These preliminary data suggest that the factors controlling the sources and sinks of NH3 indoors should be studied in greater detail, including the role of tobacco smoke.