To determine the activated clotting time (ACT) in rats and hamsters from our colony and to evaluate the response of this parameter to different heparin doses in these species, ACTs were measured using a Medtronic HemoTec ACT measurement system in samples obtained by intracardiac puncture from normal, nonanticoagulated, anesthetized rats and hamsters. Another groups of animals received different intravenous boluses of heparin to determine the dose needed to maintain ACT values > 480 sec for at least 30 min. The ACT (mean +/- SEM) was 48.0 +/- 2.17 sec for the 50 rats sampled and 42.5 +/- 2.35 sec for the 48 hamsters. Rats required a bolus of 1200 IU/kg intravenous heparin to maintain an ACT > 480 sec for 30 min; hamsters required 1000 IU/kg heparin for the same effect. We concluded that compared with humans, rats and hamsters from our colony have short ACTs and low sensitivity to heparin, in terms of the dose needed to reach a target ACT as well as the time required to sustain it. Further the ACT values in these animals showed great variability.