This paper investigates physiological and biochemical aspects of methyl halide production in rice plants over two growing seasons. Multiple separate mechanisms appear to be responsible for production of methyl halides in rice plant tissues. Evidence for multiple mechanisms is found in timing of peak emissions of methyl halides from rice, inconsistent effects of competitive inhibitors on methyl halide emissions, and large differences in methyl halide emission rates when compared to plant tissue halide concentrations. Other results show that chloride, bromide, and iodide ion concentrations in plant tissue appear to be regulated throughout the season, and observed changes in leaf tissue concentration cannot explain observed methyl halide emissions. The Km for methyl iodide formation in leaf tissue cell-free extract is 0.018 mM, suggesting a very efficient mechanism. Of the seven competitive inhibitors used, only thiol had a consistently strong effect on both methyl iodide and methyl bromide.