Haematologic parameters play a pivotal role in clinical diagnosis, the evaluation of patient before surgical intervention and in monitoring responses to therapy. The paper reviews haematologic parameters as influenced by exogenous and endogenous factors, including reproductive cycle, sex, age and season, with emphasis on domestic animals reared in the tropics. It is concluded that reproductive cycle, sex, age and season modulate haematologic parameters, and that they should be considered in order to ensure accurate interpretation of the parameters in domestic animals.