Low-temp. gas chromatog. coupled with plasma mass spectrometry (GC-ICP-MS) and at. fluorescence spectrometry methods were employed to investigate the occurrence of volatile elemental mercury and volatile dimethylmercury in environmental and biol. samples. The results demonstrate the occurrence of elemental mercury as a contaminant, along with dimethylmercury as a common trace compd., in natural gas and ambient air. In contrast to an air matrix, the absence of oxygen in the natural gas matrix ensures the stability of dimethylmercury. Elemental mercury was the only species detected in human breath, its concn. being roughly correlated with the quality and surface area of exposed amalgam fillings. However, after the removal of amalgam fillings, substantial amts. of org. mercury can be found in feces, strengthening a biovolatilization hypothesis similar to bismuth. Within the anaerobic environment of the colon, the microflora present may be able to biotransform mercury to hydride and methylated species, which can subsequently diffuse into the blood stream, and be exhaled or cross the blood/brain barrier.