1 Poly (ADP-ribose) synthetase (PARP) is a nuclear enzyme activated by strand breaks in DNA, which are caused inter alia by reactive oxygen species (ROS). Here we report on (i) a new synthesis of a water-soluble and potent PARP inhibitor, 5-aminoisoquinolinone (5-AIQ) and (ii) investigate the effects of 5-AIQ on the circulatory failure and the organ injury/dysfunction caused by haemorrhage and resuscitation in the anaesthetized rat. 2 Exposure of human cardiac myoblasts (Girardi cells) to hydrogen peroxide (H2O2, 3 mM for 1 h, n = 9) caused a substantial increase in PARP activity. Pre-treatment of these cells with 5-AIQ (1 mu M - 1 mM, 10 min prior to H2O2) caused a concentration-dependent inhibition of PARP activity (IC50: similar to 0.01 mM, n = 6). 3 Haemorrhage and resuscitation resulted (within 4 h after resuscitation) in a delayed fall in blood pressure (circulatory failure) as well as in rises in the serum levels of (i) urea and creatinine (renal dysfunction), (ii) aspartate aminotransferase (AST), alanine aminotransferase (ALT), and gammaglutamyl-transferase (gamma-GT) (liver injury and dysfunction), (iii) lipase (pancreatic injury) and (iv) creatine kinase (CK) (neuromuscular injury) (n = 10). 4 Administration (5 min prior to resuscitation of 5-AIQ) (0.03 mg kg(-1) i.v., n = 8, or 0.3 mg kg(-1) i.v., n = 10) reduced (in a dose-related fashion) the multiple organ injury and dysfunction? but did not affect the circulatory failure, associated with haemorrhagic shock. 5 Thus, 5-AIQ abolishes the multiple organ injury caused by severe haemorrhage and resuscitation.