The hypothesis that morphine acts on the serotonergic system to produce analgesia is based on the previous observations that (1) lesions and stimulation of midbrain raphe nuclei after the threshold to nociceptive stimuli; (2) morphine alters the turnover of serotonin (5-hydroxytryptamine; 5-HT). Microiontophoretic experiments were carried out to determine if morphine affected the firing rate of cells in five areas of the serotonergic system consisting of 5-HT containing neurons in the midbrain raphe nuclei (dorsal raphe and median raphe) or neurons in three areas (amygdala, optic tectum and subiculum) which are thought to receive a 5-HT input from the raphe nuclei. Morphine administered microiontophoretically slowed or inhibited the spontaneous neuronal firing in 34% of the cells studied in both the pre- and post-synaptic areas; systemically administered morphine gave similar results. However, the inhibition of neuronal firing by morphine in the five areas of the serotonergic system studied was not a specific narcotic effect because (1) it was not stereospecific; and (2) it was rarely blocked by naloxone. The inhibitory effect of morphine does not appear to be related to the analgesic effects of morphine because neuronal firing in the midbrain raphe nuclei and three possible postsynaptic areas was rarely altered by a nociceptive stimulus. It is concluded that (1) the analgesic effect of morphine is not related to an effect on neurons in the midbrain raphe nuclei and three areas which receive an identified 5-HT input; and (2) the effect of morphine on neurons in these five areas of the serotonergic system is not a specific narcotic effect.