Developmental pharmacodynamics is the study of age-related maturation of the structure and function of biologic systems and how this affects response to pharmacotherapy. This may manifest as a change in the potency, efficacy, or therapeutic range of a drug. The paucity of studies exploring developmental pharmacodynamics reflects the lack of suitable juvenile animal models and the ethical and practical constraints of conducting studies in children. However, where data from animal models are available, valuable insight has been gained into how response to therapy can change through the course of development. For example, animal neurodevelopmental models have revealed that temporal differences in the maturation of norepinephrine and serotonin neurotransmitter systems may explain the lack of efficacy of some antidepressants in children. GABA(A) receptors that switch from an excitatory to inhibitory mode during early development help to explain paradoxical seizures experienced by infants after exposure to benzodiazepines. The increased sensitivity of neonates to morphine may be due to increased postnatal expression of the mu opioid receptor. An age dependency to the pharmacokinetic-pharmacodynamic relationship has also been found in some clinical studies. For example, immunosuppressive effects of ciclosporin (cyclosporine) revealed markedly enhanced sensitivity in infants compared with older children and adults. A study of sotalol in the treatment of children with supraventricular tachycardia showed that neonates exhibited a higher sensitivity towards QTc interval prolongation compared with older children. However, the data are limited and efforts to increase and establish data on developmental pharmacodynamics are necessary to achieve optimal drug therapy in children and to ensure long-term success of pediatric drug development. This requires a dual 'bottom up' (ontogeny knowledge driven) and 'top down' (pediatric pharmacokinetic-pharmacodynamic studies) approach.