The purpose of this study was to determine the 24 h urinary excretion of a sulphur containing amino acid called felinine in entire and castrated cats of both sexes. Entire male cats excreted (mean +/- SEM) 122 +/- 23.6 mmol of felinine per kg bodyweight per day with castrated males, entire females and spayed females excreting 41 +/- 8.4, 36 +/- 7.3 and 20 +/- 3.8 mmol, respectively. There was an overall significant difference between groups in the amounts of felinine excreted in 24 h [F(3, 24) = 11.8, p < 0.0001] with there being significant differences between entire males and castrated males (p < 0.001) and castrated males and spayed females (p < 0.05). There was no difference in excretion between entire and spayed females. Urine volumes were not significantly different for the 24 h period. The differences in excretion levels were caused by different concentrations of felinine in the urine with entire male cats excreting (mean +/- SEM) 2.0 +/- 0.55 g of felinine per litre of urine. The data obtained in the present study support the concept that felinine, which has been found in Felidae species only, may be testosterone dependent. Felinine may be involved in territorial marking.