Pharmacodynamic monitoring measures biologic response to a drug, which, alone or coupled with pharmacokinetics, provides a novel method for the optimization of drug dosing. Pharmacodynamic monitoring has been investigated by us and other investigators on primarily five immunosuppressive drugs: cyclosporine (CsA), mycophenolate mofetil (MMF), rapamycin (RAPA), azathioprine (AZA), and methylprednisolone (MP). The pharmacodynamic monitoring of CsA and MMF involves measurement of the activity of the enzymes calcineurin and inosine monophosphate dehydrogenase, respectively. The pharmacodynamics of AZA are assessed by measurement of the activity of thiopurine methyl transferase (TPMT), which is induced by a metabolite of AZA, 6-mercaptopurine. The pharmacodynamics for RAPA involve the measurement of a P70 S6 kinase activity within lymphocytes, whereas that for MP involves the measurement of the endogenous synthesis of cortisol by the suppression of the hypothalamic pituitary axis. To date, the most detailed studies have been performed involving pharmacodynamic monitoring of CsA and MMF. Similarities exist in the pharmacodynamic response to CsA and MMF in patients who undergo renal transplantation. At trough concentrations in blood, both drugs result in only a 50% reduction in activity of their target enzymes; however, there is considerable interpatient variability. Throughout the dosing interval, enzyme activity parallels that of drug concentrations. Renal transplant recipients who are treated with AZA and who exhibit an increase in TPMT activity from the time of transplantation experience fewer episodes of active rejection. Renal transplant recipients who are administered MP and in whom suppression of endogenous synthesis of cortisol is greatest exhibit the least incidence of steroid-induced side effects. Additional clinical trials relating pharmacokinetics and pharmacodynamic parameters to clinical response are under way to ascertain which provides the best guide for dosing. Pharmacodynamic monitoring may provide an alternative approach to traditional drug level measurement.