This review covers current knowledge of the pharmacodynamics and pharmacokinetics of drugs in avian species. Special attention has been paid to inter-species differences in relation to metabolic elimination, anatomy and physiology of the digestive and respiratory system, and differences in drug distribution. Intra-species differences attributable to physicochemical aspects of the drug preparation and physiological conditions of the avian patient can also influence drug efficacy. The consequences of the choice of a particular method of drug administration on pharmacokinetics are also considered.