The induction of the mixed function oxidase (MFO) system, commonly used as a biochemical marker for xenobiotics in recent years, 1-4 is also affected by normal physiological changes. The use of this as an index of stress has contributed to our ability to detect and understand the significance of exposure to xenobiotics (both organochlorines and polynuclear aromatic hydrocarbons) in the environment. 4-7 Nevertheless, several intrinsic factors such as hormonal changes during the breeding cycle significantly modify the activity of monooxygenases. 8-11 The aim of this paper is to examine, using four specific examples of studies in wild birds, the role of the sexual cycle in the modification of MFO activity and consequently the importance of considering this aspect in planning biomonitoring.