The postmortem stability of the rat Atrial Natriuretic Peptide (ANP) has been studied as a necessary and previous step to be applied in the forensic field as a postmortem marker. This peptide--whose extreme sensitivity to slight changes in blood volume is well known--could have great importance in thanatochemistry to establish a correct diagnosis when macroscopical observations and classical parameters are not conclusive or cannot be employed. The results show high stability in atrial tissue, where values are similar from 0 hours (108.99 pm/mg) to 8 hours (109.41 pm/mg) and decrease uniformly until 15 pm/mg) at 32 hours, time of our last determination. Blood ANP showed similar stability from 0 h (105.43 pg/ml) to 8 h (106.62 pg/ml).