Felinine (2-amino-7-hydroxy-5,5-dimethyl-4-thiaheptanoic acid) has been identified in the urine of several members of the Felidae family including the cat (Felis catus). Rates of excretion of 95 mg/day have been recorded for entire male cats with entire female cats excreting around 19 mg/day. These high excretion rates in entire male cats may have a significant effect on the daily sulphur amino acid requirement. The isoamyl moiety of felinine seems to originate from the same isoprenoid pool as used for the synthesis of cholesterol in the cat. The sulphur in the felinine molecule appears to originate from cysteine, although some contradictory evidence exists. The site of synthesis and the method of transportation in the blood remain largely unknown. The biological significance of felinine to the animal is still a matter for speculation, but its function as a precursor to a pheromone seems likely. Recently, an accurate chemical assay for felinine has been developed that will allow investigation of felinine in different tissues and excretions and from a wider range of mammals.