Treatment of chickens, ducklings and turkey poults with furazolidone (0.04% w/w, in the feed) for 10 days inhibited monoamine oxidase (MAO) activities in the alimentary tract, heart and brain. Only in the duckling was an inhibition of MAO activity found in the liver. In the chicken and duckling, furazolidone (200 mg/kg) by crop tube inhibited MAO activities in the duodenal mucosa, liver, heart and brain, whereas the same dose of the drug given intramuscularly to the chicken was ineffective. Furazolidone by crop tube was about equipotent as an MAO inhibitor in chickens, ducklings and turkey poults. Pretreatment of chickens and ducklings with neomycin to suppress the alimentary microflora reduced the effect of furazolidone on MAO activity, suggesting that the microflora transformed the drug to an active metabolite which subsequently inhibited MAO activities in other organs Furazolidone (0.04% w/w, 10 days) increased the amount of 5-hydroxytryptamine (5-HT) in the brain of chickens and ducklings, and potentiated the vasopressor action of tyramine in chickens. The amounts of noradrenaline and adrenaline in the brain were unaffected by the treatment, as were the actions of noradrenaline, 5-HT, histamine and tryptamine on blood pressure. In poultry, furazolidone (0.04% w/W, 10 days) produced an anorexia and an increase in the concentrations of pyruvate and lactate in blood. The activation of transketolase by thiamin pyrophosphate was greater in haemolysates from furazolidone-treated chickens than in control preparations. These observations suggested a deficiency of thiamin in the furazolidone-treated birds. However, intramuscular administration of thiamin HC1 (84 yg/kg, twice daily for 10 days) to the chicken did not prevent the development of signs characteristic of its deficiency. In chickens given furazolidone (0.04% w/w, 10 days), the weights of the adrenal and thyroid glands (as % of body weight) and the oxygen consumption of hologenized heart were all increased. Furazolidone (0.04% w/w, 10 days) did not produce an anaemia or affect the arterial blood pressure in the conscious chickens, and the activities of plasma aspartate transaminase and hepatic aminopyrine demethylase were unaffected. Ducklings survived when furazolidone was given in the feed at a concentration of 0.04% w/w for 10 days, or by crop tube at a dose of 200 mg/kg.