d-Amphetamine administered to five day old chicks provoked wing droop, postural changes and twittering. p-Cl-phenylalanine methyl ester (H 69/17) which lowered the content of 5-HT and 5-HIAA in the brain to about 30% of control values in chicks of the same age, induced no marked behavioural changes. Pretreatment with H 69/17 protected against the above mentioned effects of d-amphetamine. Simultaneously d-amphetamine induced marked excitation with aggressive behavioural components. It is concluded a) that the predominant actions of d-amphetamine in newly hatched chicks are mediated via a serotonergic (tryptaminergic) mechanism, which masks the excitatory effects of the drug in this animal species, and b) that there are great similarities, between the behavioural changes seen after d-amphetamine in 5-HT depleted chicks and imipramine in normal chicks.