1 This study aimed (1) to measure the whole blood to plasma (WB:P) and red blood cell to plasma (RBC:P) concentration ratios of propranolol in healthy volunteers and two types of patients, and (2) to compare the concentration ratios of the lipophilic drug propranolol with moderately lipophilic pindolol and hydrophilic atenolol. 2 There was no significant difference between the WB:P and RBC:P ratios of propranolol concentration in healthy volunteers and neurological patients compared with hypertensive patients. The mean +/- s.d. WB:P ratios of propranolol concentration in the three groups were 0.74 +/- 0.03, 0.71 +/- 0.05, and 0.76 +/- 0.08 respectively. The mean RBC:P ratios were 0.39 +/- 0.08, 0.36 +/- 0.11, and 0.47 +/- 0.15 respectively. WB:P and RBC:P concentration ratios of propranolol were linearly correlated with the free fraction of drug in plasma. Propranolol was 90% bound in plasma. 3 The mean WB:P and RBC:P ratios of pindolol in seven volunteers were 0.69 +/- 0.08 and 0.37 +/- 0.14 respectively. Pindolol was 71.4 +/- 8.6% bound to plasma proteins. The concentration of pindolol in the RBC was linearly correlated with that unbound in plasma. 4 In four healthy volunteers, the mean WB:P concentration ratio of atenolol was 1.07 +/- 0.25 and the mean RBC:P ratio was 1.15 +/- 0.55. 5 The similarity of the RBC:free plasma drug concentration ratios for all three drugs suggests that the use of organic solvent partition coefficients for the prediction of in vivo distribution may be unreliable.