To clarify the effect of thiamine on serotonin-related metabolism in magnesium-deficient rats, in vivo and in vitro experiments were carried out. In thiamine-excess magnesium-deficient and thiamine-adequate magnesium-deficient rats, serotonin oxidation activity in liver and stomach decreased significantly, while a decrease was not observed in thiamine-deficient magnesium-deficient rats. In vitro addition of a metal chelating agent and either thiamine or thiamine pyrophosphate to brain and liver mitochondria of intact rats resulted in a decrease in serotonin oxidation activities. Thus it is assumed that excess thiamine coupled with magnesium deficiency plays a role in the inhibition of serotonin oxidation and consequently serotonin increase.