Serum samples from patients on phenytoin (PHT), carbamazepine (CBZ), or phenobarbitone (PB) monotherapy were filtered at 15, 25, and 37 degrees C and the free concentrations measured by high-performance liquid chromatography. The mean apparent dissociation constants at each temperature were calculated, and were used to predict free drug levels from a further series of patients' samples in which total drug and albumin concentrations only were known. The correlation coefficients (r) between these predicted free levels and experimental results obtained by analysis of the same samples for PHT, CBZ, or PB were 0.977, 0.968, and 0.998, respectively, at 25 degrees C; at 37 degrees C, the corresponding values of r were 0.975, 0.961, and 0.997, respectively. We then determined free fractions (alpha) of PHT, CBZ, and PB at 25 and 37 degrees C and used these values to derive theoretical target ranges for free levels for each of the three drugs. We discuss the implication of these results for patient care, with special reference to the need to specify temperature and quote the appropriate target range when analyses of free levels of AEDs are carried out.