To investigate the effect of Roux-en-Y gastric bypass on the rate and extent of absorption of metoprolol and its main active metabolite α-OH metoprolol, after a single oral dose of 95 mg metoprolol CR tablet in 10 female bariatric surgery patient volunteers, before and after surgery. The pharmacokinetic parameters that will be determined are: - Cmax The maximum serum concentration of metoprolol. - Tmax The time after oral administration of metoprolol when the maximum serum concentration is reached. - AUC t=0-24 The area under the serum concentration-time curve, a measurement of the bioavailability of metoprolol until 24 hours after intake. The effect can be determined according to the following ratios as listed in the studyprotocol.