The present study sought to determine whether rats, treated neonatally with 5,7-dihydroxytryptamine (5,7-DHT), have an increased sensitivity to the respiratory and cough-depressant effects induced by dihydrocodeine. The serotonin (5-HT) levels in the whole brain of 5,7-DHT-treated rats were reduced to 19% of the corresponding control values. The 5,7-DHT-treated rats were supersensitive to the depression in frequency of respiration and cough reflex produced by i.p. administration of dihydrocodeine. The increased sensitivity to dihydrocodeine in terms of the depression of frequency of respiration and the cough reflex in 5,7-DHT-treated rats could possibly have been due to changes in the sensitivity of serotonergic receptors.