A blind, parallel, prospective, clinical study was conducted to investigate the effect of ascorbic acid on human serum hydrocortisone concentrations which were decreased by the administration of cimetidine. The study population included 16 male adults scheduled for major abdominal vascular surgery. The study was conducted in surgical patients under anaesthesia, in which steroidogenesis was inhibited by cimetidine. The results showed a reduction in serum hydrocortisone concentrations in patients receiving a placebo. In patients receiving ascorbic acid, there was a significant increase in serum hydrocortisone concentration. This reflects the normal serum hydrocortisone profile for this operation and anaesthetic technique. Cimetidine can bind to cytochrome P-450 covering the active haem group, the cytochrome proves to be of vital importance for hydroxylation reactions, involved in human steroidogenesis. Serum hydrocortisone concentrations will decrease when cytochrome P-450 becomes blocked. Intravenous administration of ascorbic acid was supposed to cause relief for this decrease. The reasons are undetermined yet. This investigation proved that ascorbic acid can prevent cimetidine-induced decrease of human serum hydrocortisone concentrations.