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Gastrointestinal (GI) complications after cardiac surgery have a substantial impact on outcome. The aims were to investigate the frequency of, and methods for detecting, GI dysfunction after cardiac surgery and its relation to outcome, and the impact of vasoactive drugs on the GI tract in experimental cardiogenic shock. Paper I investigated the intraabdominal metabolism, using intraperitoneal microdialysis, during and after routine cardiac surgery in six patients. The results imply that, even during a normal perioperative course, the GI tract may be subjected to a subclinical anaerobic state. In Paper II the impact of stepwise reductions of cardiac output (CO) on the metabolism and circulation in the GI tract was studied in anaesthetised pigs using cardiac tamponade (n=6) or partial inflation of a caval vein balloon (n=6). The two models had similar haemodynamic effects and the intraabdominal metabolism became increasingly anaerobic when the CO was reduced by 50%. In Paper III the caval vein balloon model was utilised to examine the GI effects of two inodilators (levosimendan and milrinone) and two vasoconstrictors (vasopressin and norepinephrine) at 40% CO reduction (n=7/group). Negligible splanchnic vasodilation by the inodilators in fixed low CO, and possible GI specific side effects of high dose vasopressors, were demonstrated. Paper IV included 501 cardiac surgery patients assessed using the Acute Gastrointestinal Injury (AGI) grade. Only 32.7% were asymptomatic during the first three postoperative days. At least GI dysfunction, i.e. AGI grade ≥2, developed in 2.2% and was associated with more complex surgeries and higher postoperative frequencies of GI complications and mortality. In Paper V a biomarker for enterocyte damage, intestinal fatty acid-binding protein (IFABP), was investigated in relation to AGI grade. The group with AGI ≥2 (n=11) was compared to a matched group without GI symptoms (n=22). An I-FABP concentration in the fourth quartile on day one was associated with higher frequencies of organ dysfunction and 365-day mortality. In conclusion, this thesis provides evidence for an association between intraoperative GI injury, postoperative GI dysfunction and manifest complications, and that the effects of inodilators and vasoconstrictors must be considered.