Characteristics of hydroxyindole-O-methyltransferase (HIOMT) activity were examined in pineal gland tissue from 10 muskoxen shot by native hunters in November, 1990. The enzyme preferentially methylated N-acetylserotonin, with other hydroxyindole compounds showing relatively low affinities; activity peaked sharply at pH 8.2. HIOMT was noncompetitively inhibited by its substrate, N-acetylserotonin, and competitively inhibited by its product S-adenosylhomocysteine. The catalytic mechanism appeared to be ordered as described in previous studies: S-adenosylmethionine was the obligatory first substrate, followed by N-acetylserotonin; methyl transfer then occurred and the products, melatonin and S-adenosylhomocysteine, were released sequentially. Interestingly, the inhibition constant (Ki) for N-acetylserotonin was relatively close to the Michaelis-Menten constant (Km), which might allow physiological concentrations of N-acetylserotonin to inhibit HIOMT activity in vivo. This effect could be relevant to the ecology of free-living muskoxen during the dramatic seasonal fluctuations in dietary protein and daily photoperiod associated with their arctic habitat.