We investigated whether postweaning protein malnutrition (PM) affects serotonergic systems. Mice were fed a PM diet or normal protein (control) diet from weaning (21 d of age). Twenty days later, we tested for behavioral effects of the selective serotonin (5-HT)(1A) receptor agonist 8-hydroxy-N,N-dipropyl-2-aminoteralin (8-OH-DPAT) and the 5-HT releaser d-fenfluramine. The number of head weaving responses induced by 8-OH-DPAT or d-fenfluramine in the PM mice was significantly increased compared with the control diet group. The effects of 8-OH-DPAT and d-fenfluramine were blocked by pretreatment with the selective 5-HT(1A) receptor antagonist WAY-100635 (0.01 mg/kg). However, postpubertal (56 d of age) mice fed with the PM diet did not show an enhancement of the 8-OH-DPAT-induced head weaving response. These results indicate the occurence of a supersensitivity of postsynaptic 5-HT(1A) receptor in the postweaning PM group. Moreover, they highlight the postweaning stage as a vulnerable period to malnutrition-induced alterations in central serotonergic systems.