The levels of ash deposition and soil heating encountered in vegetation fires were simulated in two soils to determine the relative effects of ash, heat, and ash-heat interaction on respiration rate (O2 uptake, CO2 evolution). In sandy podzolic soil respiration rate generally increased with rate of ash addition. For the more highly buffered krasnozem soil, high rates of ash slightly decreased respiration rate, while small amounts had no effect. The chemical composition of ash determines its effects on soil respiration, and results from ash; (a) increasing soil pHper se, (b) adding a small amount of organic carbon to the soil and (c) changing the solubility in water of native soil organic carbon. Ash did not stimulate respiration rate in steam sterilised podzolic soil, indicating that ash acts via its influence on active soil biological populations. Mild heating had little effect on respiration rate in either soil, but a positive interaction between heat and ash increased respiration in podzolic soil. These studies demonstrate the potential of heat and ash to alter soil biological activities in the field. The significance of such changes after fire to plant nutrition needs to be evaluated for each particular ecosystem where fire is an important environmental factor.