Early life adversity is associated with an increased incidence of psychiatric illness in adulthood. Although the mechanisms underlying this association are unclear, one possible substrate is brain 5-hydroxytryptamine neurotransmission, which is reportedly abnormal in several psychiatric disorders. This study examined the effect of a rat model of early life adversity, early maternal separation, on 5-hydroxytryptamine neurotransmission in adulthood. In vitro electrophysiological experiments revealed that, in early maternal separation rats compared with controls, the sensitivity of alpha1-adrenoceptors on 5-hydroxytryptamine neurons in the dorsal raphe nucleus was significantly reduced, whilst the sensitivity of 5-hydroxytryptamine1A receptors showed a nonsignificant trend to reduction. In in vivo microdialysis experiments, the 5-hydroxytryptamine1A receptor agonist-induced suppression of 5-hydroxytryptamine release in the frontal cortex was reduced in early maternal separation animals, suggesting desensitization of 5-hydroxytryptamine1A autoreceptors. There was no increase in basal 5-hydroxytryptamine in the frontal cortex as measured by microdialysis and a nonsignificant trend towards increased basal firing activity of classical (non-bursting) 5-hydroxytryptamine neurons in the dorsal raphe nucleus measured by in vivo electrophysiology. Finally, early maternal separation failed to alter expression of messenger ribonucleic acids coding for 5-hydroxytryptamine1A or alpha1B receptors in the dorsal raphe nucleus as measured by in situ hybridization histochemistry, suggesting that functional changes in receptor sensitivity observed are not due to changes in receptor gene transcription. The findings demonstrate that early life adversity programs changes in sensitivity of the two principal regulators of 5-hydroxytryptamine neuronal activity. Similar effects in humans may contribute to the increased incidence of psychiatric illness in individuals exposed to early life adversity.