Evidence suggests that cocoa from the bean of Theobroma cacao L. has beneficial effects on cardiovascular disease. The aim of this study was to investigate if cocoa extract and dark chocolate influence angiotensin-converting enzyme (ACE) and nitric oxide (NO) in human endothelial cells (in vitro) and in healthy volunteers (in vivo). ACE activity was analyzed with a commercial radioenzymatic assay and measured in human endothelial cells from umbilical veins (HUVEC) after 10 minutes of incubation with cocoa extract. NO was measured after 24 hours of incubation. ACE activity and NO were measured at baseline and after 30, 60, and 180 minutes in 16 healthy volunteers after a single intake of 75 g of dark chocolate containing 72% cocoa. Significant inhibition of ACE activity (P < 0.01) and significant increase of NO (P < 0.001) were seen in HUVEC. In the study subjects, a significant inhibition of ACE activity (mean 18%) 3 hours after intake of dark chocolate was seen, but no significant change in NO was seen. According to ACE genotype, significant inhibition of ACE activity was seen after 3 hours in individuals with genotype insertion/insertion and deletion/deletion (mean 21% and 28%, respectively). Data suggest that intake of dark chocolate containing high amount of cocoa inhibits ACE activity in vitro and in vivo.