AbstractNeither the administration of sinapine bisulphate in the diet, nor the repeated intramuscular or intravenous injection of large doses of this substance, reduced the ability of chicks or laying hens to oxidise trimethylamine (TMA), as measured by the activity of TMA oxidase in hepatic microsomes or the amounts of 14C‐TMA oxide that appeared in the plasma after the infusion of a standard dose of 14C‐TMA. Thus, in contrast to its behaviour in vitro, sinapine did not produce significant inhibition of TMA oxidase under these conditions. Since the amount included in the diet was similar to that which would be provided by a diet containing 10% rapeseed meal, it was concluded that sinapine is not involved in the depression of TMA oxidation that occurs when the meal is fed and acts solely as a source of TMA in the consequent production of egg taint.