Fel d I is produced by salivary and sebaceous glands. Hormonal control of sebum production is clearly established. The influence of cat castration and supplementary treatment with testosterone on the production of sebum and Fel d I in cat skin have been researched in this study. On day 1, 12 male cats were anaesthetized and three skin areas carefully shaven. Then the level of lipids on skin surface was measured by means of a photometric method. Finally, the three areas of skin were washed with 5 ml of distilled water through a plastic cylinder. Fel d I collected in the washes was measured with a two-site monoclonal antibody based ELISA. On day 2, six cats were castrated, the other six were used as a control group. Two and 4 weeks later, the levels of lipids and Fel d I in skin washes were measured again in all cats. On day 30, the six castrated cats were injected intramuscularly with prolonged-action testosterone. Two weeks later, quantification of lipids and Fel d I in all animals was repeated. Sebum and Fel d I levels decreased in all castrated animals. Injecting the castrated cats with testosterone led to a significant increase in sebum and Fel d I production. Our findings indicate that Fel d I production is influenced by the production of hormones.