Laboratory experiments were undertaken to examine the effects of high soil temperatures on N biotransformations in sandy soils. Soils were incubated at 30 degrees, 40 degrees, 50 degrees, and 60 degrees C for 2 days, before all treatments were kept at 30 degrees C for up to 41 days. Another laboratory experiment evaluated the effect of different cycles of exposure to 50 degrees and 30 degrees C, including frequency and duration of exposure to 50 degrees C, to assess the sensitivity of N biotransformations to temporary increases in temperature in the high range. CO(2)-C production, soil microbial biomass-C, gross N mineralisation, gross N immobilisation, and potential gross nitrification were measured. Gross N mineralisation and CO(2)-C production increased with temperature (in the range 30 degrees-50 degrees C) and exhibited a Q(10)-relationship close to 2. Between 50 degrees and 60 degrees C, Q(10) was closer to 2.8. The increase in gross N mineralisation and CO(2)-C production after exposure to 50 degrees and 60 degrees C is attributed to the decomposition of dead microbial biomass by the viable microbial population but this flush in activity was short-lived. Immobilisation rate was always low and remained unaffected by the temperature regime, probably because the growth of the microbial biomass was inhibited at the higher temperatures. This imbalance between gross N mineralisation and immobilisation resulted in rapid increases in mineral N in soil. Two 6-h cycles of 50 degrees C interspersed with 30 degrees C were equally as effective as a single 48-h exposure at stimulating CO(2) production. Evidence of uncoupling CO(2) production and gross N mineralisation was observed in one study where soil was incubated at 50 degrees C, but this response was not universal. The nitrification process was totally suppressed by exposure to temperature higher than 40 degrees C, probably due to thermal denaturation of enzymes. The relevance of findings to field conditions is discussed.