The microbial activity in a Rhodic eutrudox (R), a Typic eutrudox (V) and a Quartzipsamment (Q) was monitored by respirometric and calorimetric methods. CO2 evolution was monitored for 98 days by titrimetry and conductimetry for control amended samples (A) with 25% of cattle manure (E), municipal refuse compost (L), earthworm casts (H) or 1.25 kg ha(-1) of trifluralin (T). Average values of all treatments through respiration at the end of the incubation period were 5.24 +/- 0.34, 6.13 +/- 0.31 and 6.50 +/- 0.33, in Mg CO2 g(-1) soil, for R, V and Q, respectively, by titrimetry and 8.89 +/- 0.44, 10.41 +/- 0.54 and 10.41 +/- 0.52, in Mg CO2 g(-1) soil, for R, V and Q, respectively, for conductimetry. Excellent correlation (r = 1.00) between titrimetry and conductimetry was observed. The decreasing order for respiration was E, H, L and T. After each incubation time, the conductimetric values were higher than those for titrimetry, for all treatments of these Brazilian soils. Average values of the exothermic thermal effect were: 0.58 +/- 0.02, 0.60 +/- 0.02 and 0.67 +/- 0.01 kJ g(-1) soil, for R, V and Q, respectively, for 103 days. A significant correlation coefficient of 0.91 and P < 0.0001 between calorimetric and respirometric values over 98 days was observed. Based on the obtained calorimetric results, it can be proposed that this technique should be as a useful analytical method for determining the microbial activity in soils. (C) 2003 Elsevier B.V. All rights reserved.