The standard method for cimetidine analysis employs a three-step extraction procedure and metiamide as the internal standard. This assay requires 5.0 ml of whole blood and has a 6 min analysis time after injection onto the column. Clinical studies of cimetidine in seriously ill patients or children require smaller sample volumes and often more rapid turnaround. The present high pressure liquid chromatographic (HPLC) procedure is a modification which re-quires only 0.5 ml of serum and is almost two times faster, as injections can be made every 4.1 min. This assay uses a Dupont SIL column, a similar but scaled-down extraction procedure, and similar chromatographic conditions. In addition to cimetidine, the assay also measures creatinine in all samples. Absolute recovery for cimetidine was 53% of added amount, compared with 63% by the previous method. For creatinine, the absolute recovery was 73%. A comparison of HPLC creatinine concentrations in 28 patient samples, with values measured by autoanalyzer, demonstrated good agreement between the methods (r = 0.94). The assay for cimetidine and creatinine has been applied to serum, bile, pancreatic fluid, gastric juice, pleural fluid, cerebrospinal fluid, and ascitic fluid. Concentrations in all body tissues can also be measured, provided that tissue homogenates are first subjected to an aqueous extraction. This cimetidine assay has shown excellent performance over one year of use in studies of seriously ill patients. To date, the only substance found to interfere with this assay is trimethoprim. High specificity, microcapabilities, and rapid analysis time make this modification especially suitable for clinical pharmacokinetic management of seriously ill patients given cimetidine.