Oscillations of gene expression and physiological activity in suprachiasmatic nucleus (SCN) neurons result from auto-regulatory feedback loops of circadian clock gene transcription factors. In the present experiment, we have determined the pattern of PERIOD1, PERIOD2 and CLOCK expression within neuroendocrine dopaminergic neurons (NDNs) of ovariectomized (OVX) rats. We have also determined the effects of per1, per2 and clock mRNA knockdown in the SCN with antisense deoxyoligonucleotides (AS-ODN) on DA release from NDNs. Diurnal rhythms of PER1 and PER2 expression in tuberoinfundibular dopaminergic (TIDA) and periventricular hypophyseal dopaminergic (PHDA) neurons, peaked at CT18 and CT12, respectively. Rhythms of PER1 expression in tuberohypophyseal dopaminergic (THDA) neurons were undetectable. Rhythms of PER2 expression were found in all three populations of NDNs, with greater levels of PER2 expression between CT6-CT12. AS-ODN injections differentially affected DA turnover in the axon terminals of the median eminence (ME), neural lobe (NL) and intermediate lobe (IL) of the pituitary gland, resulting in a significant decrease in DA release in the early subjective night in the ME (TIDA), a significant increase in DA release at the beginning of the day in the IL (PHDA) and no effect in the NL (THDA). AS-ODN-treatment induced a rhythm of DA concentration in the anterior lobe, with greater DA levels in the middle of the day. These data suggest that clock gene expression, particularly PER1 and PER2, within NDNs may act to modulate diurnal rhythms of DA release from NDNs in the OVX rat.