Background: The impact of bariatric surgery on abnormalities in blood lipids and lipoprotein metabolism during the perioperative period has been poorly studied. Objective: We studied the impact of bariatric surgery on the composition and quantitative properties of lipoproteins and the activity of lipases in the plasma of perioperative morbidly obese patients. Methods: We examined the plasma lipoproteins and lipolytic activities of 34 morbidly obesepatients one month before surgery (OB), pre-anaesthesia (-S), post-anaesthesia ( S), and one day and one month after open Roux-en-Y gastric bypass (RYGB) surgery. Results: Surgical injury induced acute stress, as evidenced by transitory hyperglycaemia and elevated plasma levels of stress hormones. Lipid profiles revealed a significant reduction during surgery and the day after in the plasma levels of total cholesterol (p < 0.0001), which was mainly due to a decrease in low-density lipoprotein cholesterol (cLDL) and was confirmed with a significant reduction in the plasma levels of LDL (approximately 26% reduction). Significant (p < 0.0001) changes were detected in the plasma levels of high-density lipoprotein cholesterol (cHDL) as well as a significant decrease (approximately 19% reduction) in the plasma levels of HDL. A significant (p < 0.0001) rise was noted in the plasma levels of both Lipoprotein Lipase (LPL) (approximately 2.6-fold increase) and hepatic lipase (HL) (approximately 2.2-fold increase) on the day after surgery, occurring simultaneously with the maximum increase in C-reactive protein (CRP) and a day after the peak values for non-esterified fatty acid (NEFA), adrenocorticotropin hormone (ACTH), cortisol and glucose. Conclusion: The present study reveals unreported quantitative perioperative changes in plasma lipases and lipoproteins and related metabolic determinants that may contribute to the adaptive metabolic response to RYGB-induced stress.