BACKGROUND Previous experiments have shown that in vivo Fel d 1 production is under hormonal control. It strongly decreased 1 month after castration of male cats and increased after testosterone injection. OBJECTIVE These results led us to put forward the hypothesis that Fel d 1 production could be more important in male that in female cats. Five adult male and five adult female cats were studied. METHODS On day 0 three separate sites of each cat's trunk were shaved, and a 5.72 cm2 area in each site was washed twice (D0a and D0b) with 5 ml of distilled water. Then a collar was attached to the neck to prevent contamination from saliva. Twenty-four hours later on day 1, all areas were washed again once (D1a). Skin washes and fur extracts from the shaved areas were evaluated for Fel d 1 content. The same procedures (apart from collection of fur) were repeated 5 months later. RESULTS The Fel d 1 level was higher in the first skin wash of male cats compared with that of female cats on each separate site of the trunk in both experiments, initially and 5 months later. Results were significant in the first experiment (median Fel d 1 per milliliter: 69.4 and 28.9 mU, respectively, for the combined three sites; p < 0.05). Fel d 1 production over a 24-hour period was higher in male cats, but the difference did not reach statistical significance. Furthermore, correlation between Fel d 1 levels in washes D0a and in fur was highly significant (p < 0.01). CONCLUSIONS These results confirm that Fel d 1 originates from skin. Furthermore, they suggest that Fel d 1 production is higher in male than in female cats.