Myocardial ischemia-reperfusion injury increases both tissue levels and release of histamine. To study the possible effects of ischemia-reperfusion on histamine metabolism tissue activities of histidine decarboxylase (HDC), histamine N-methyl transferase (HNMT) and diamine oxidase (DAO) were investigated in isolated rat hearts subjected to either 20 min global ischemia and 40 min reperfusion (n = 10) or control perfusion (n = 8). Histamine in the coronary effluent increased from 21 +/- 4 nmol/min (mean +/- SEM) before ischemia to 55 +/- 5 and 50 +/- 7 nmol/min after 4 and 10 min reperfusion (p < 0.004 and p < 0.004). Tissue HDC activity did not change during observation in any group. HNMT activity was unchanged in controls, but increased from 0.37 +/- 0.04 to 0.84 +/- 0.18 and 0.96 +/- 0.22 pmol methylhistamine/mg protein hour after 4 and 10 min reperfusion (p < 0.008 and p < 0.01). DAO decreased similarly in controls and ischemic-reperfused hearts during observation. In conclusion, the previously observed increase of tissue histamine during reperfusion cannot be explained by increased histamine synthesis or decreased histamine catabolism.