Three reversed-phase high performance liquid chromatography (HPLC) methods with UV detection were developed and fully validated for the quantification of three beta-blockers: atenolol, metoprolol and propranolol. After validation, error structures for the HPLC analysis were established using a convenient and practical procedure. The mean percentage of relative standard deviation (RSD) of the experimental concentrations (C), were less than 4.29% for proportionality and less than 3.68% for precision for any of the drugs, which allowed the quantitation of beta-blockers assayed at concentrations in the range 25-0.78 micrograms.ml-1. The error structures for the HPLC analysis were: SD (micrograms.ml-1) = 5.02 x 10(-2) C for atenolol, SD (micrograms.ml-1) = 4.55 x 10(-2) + 0.63 x 10(-2) C - 7.58 x 10(-6) C3 for metoprolol and SD (micrograms.ml-1) = 2.73 x 10(-2) C - 3.49 x 10(-4) C2 for propranolol. The reciprocal of the square of the SD of the drug concentrations measured within the calibration curve could be used as weighting methods in parameter estimation by non-linear regression.