The development of an optimum dosage regimen, one which balances patient convenience with the proper body content of a drug, is an essential consideration for rational therapy with a drug product. The complexity of the equations employed for dosage regimen calculations is dependent upon the complexity of the pharmacokinetic model describing the situation. If the indicated dosage regimen for a drug is 4 times a day, it is unlikely that it will be taken once every 6 hr. A narrow range of safety together with increased accumulation due to renal failure mandate an adjustment of the dosage regimen. The most frequently employed approach is to maintain the same average steady-state plasma concentration Cavss during renal insufficiency as that achieved with the usual dosage regimen and normal renal function. A single oral dose of tetracycline is found to give a linear terminal semilog plot for total drug in blood versus time.