To investigate the effects of an endogenous release of serotonin (5-HT) in the lamprey spinal cord, in vitro, the 5-HT uptake-blocker citalopram (1-10 microM) was added to the bathing solution. Samples taken from the physiological solution through high-performance liquid chromatography (HPLC) showed that 5-HT was released from the spinal cord. To study the effect of this endogenous release of 5-HT on the spinal network generating locomotion, 'fictive locomotion' was induced by bath application of N-methyl-D-aspartate (NMDA, 100 microM). It elicited a steady locomotor rhythm between 0.2 and 2.5 Hz. The effects of citalopram were the following: (1) the locomotor frequency slowed down, (2) the intensity of the ventral root bursts was increased and (3) the intersegmental phase lag was prolonged. The effects of citalopram and thus presumably of an endogenous release of 5-HT were similar to what has previously been observed during bath application of 5-HT. In the latter case the conditions are different, since all 5-HT receptors regardless of their location in relation to the 5-HT-containing boutons will be affected in a similar way.