This study evaluated the effects of batch size on the in vitro dissolution and the in vivo bioavailability of immediate release formulations of propranolol hydrochloride and metoprolol tartrate. The formulations were manufactured as small and large batches (6 kg and 60 kg for propranolol; 14 kg and 66 kg for metoprolol), and dissolution was performed using USP Apparatus I at 100 rpm and pH 1.2. Two panels of 14 subjects each were randomly assigned to receive the small and large batches of either propranolol or metoprolol in an open, randomized single-dose study. Blood samples were collected over a 24-hour (propranolol) or 18-hour (metoprolol) period and analyzed by validated methods. As determined by the f2 metric (similarity factor), the dissolution of the small and large batches of propranolol and metoprolol was similar. The mean Cmax and AUC(inf) for the small batch of propranolol were 79.0 microg/L and 536 microg/L/hr, and for the large batch they were 83.5 microg/L and 575 microg/L/hr. Cmax and AUC(inf) for the small batch of metoprolol were found to be 95.5 microg/L and 507 microg/L/hr and for the large batch, 95.1 microg/L and 495 microg/L/hr. The 90% confidence intervals for the small and large batches were within the 80% to 120% range for lnCmax, and lnAUC(inf) for both the propranolol and metoprolol formulations. These results suggest that the scale-up process does not significantly affect the bioavailability of highly soluble, highly permeable drugs and in vitro dissolution tests may be useful in predicting in vivo behavior.