There is increasing evidence both from 'association' and 'intervention' studies that iron deficiency has an adverse effect on brain function in animals and children. The severity and duration of iron deficiency are important in determining the effect on development. Iron replacement therapy has immediate (within 14 days) and long-term (over 3 months) beneficial effects on behaviour and psychomotor development. The mechanisms for this probably involve a number of biochemical pathways in which iron is essential. These include mitochondrial enzymes and various neurotransmitters. Cytochrome C is reduced by iron deficiency but brain tissue is relatively spared until the deficiency is severe. Levels of neurotransmitters such as noradrenaline, serotonin and dopamine are all altered during iron deficiency and this may explain some of the behavioural and developmental changes that occur.