We studied the release of endogenous norepinephrine (NE) from rat hypothalamic slices. The slices were superfused with Krebs-bicarbonate buffer and field-stimulated with 960 or 1800 pulses applied 40 min apart. The spontaneous and stimulation-evoked overflow of NE into the medium were both enhanced by addition of a NE reuptake inhibitor, desipramine (10 microM). Repeated stimulations in the presence of desipramine decreased the evoked release of NE during the second and third stimulation periods (S2 and S3). The attenuation of NE release during S2 was not reversed by the addition of tyrosine (50 microM) to the medium.