Because methionine enkephalin contributes to and dynorphin opposes dilation during a 10-min hypoxic exposure, opioids modulate pial artery dilation to this stimulus. However, such modulation may be dependent on the duration of hypoxia. The present study was designed to characterize the modulation of hypoxic pial dilation by opioids as a function of stimulus duration in newborn pigs equipped with a closed cranial window. Hypoxic dilation was decremented in both moderate and severe groups (PO₂  35 and 25 mmHg, respectively) during 20-min and 40-min exposure periods compared with the response during 5 or 10 min of stimulation (24 ± 1, 25 ± 1, 18 ± 1, and 14 ± 1% for 5, 10, 20, and 40 min of moderate hypoxia; means ± SE). Moderate and severe hypoxia had no effect on cerebral spinal fluid (CSF) methionine enkephalin or dynorphin concentration during a 5-min exposure period. During a 10-min exposure, however, both opioids were increased in CSF. During 20- and 40-min exposure periods, CSF dynorphin continued to increase, whereas methionine enkephalin steadily decreased (962 ± 18, 952 ± 21, 2,821 ± 15, 2,000 ± 81, and 1,726 ± 58 pg/ml methionine enkephalin for control, 5, 10, 20, and 40 min of moderate hypoxia, respectively). The µ-opioid (methionine enkephalin) antagonist -funaltrexamine had no influence on dilation during the 5-min exposure, decremented the 10- and 20-min exposures, but had no effect on 40-min exposure hypoxic dilation. Whereas the -opioid (dynorphin) antagonist norbinaltorphimine similarly had no effect on a 5-min exposure dilation, it, in contrast, potentiated 10-, 20-, and 40-min exposure hypoxic dilations (23 ± 1 vs. 23 ± 1, 24 ± 1 vs. 32 ± 1, 16 ± 1 vs. 24 ± 2, and 13 ± 1 vs. 23 ± 3% for 5, 10, 20, and 40-min hypoxic dilation before and after norbinaltorphimine). These data show that opioids do not modulate hypoxic pial dilation during short but do so during longer exposure periods. Moreover, hypoxic pial dilation is diminished during longer exposure periods. Decremented hypoxic pial dilation during longer exposure periods results, at least in part, from decreased release of methionine enkephalin and accentuated release of dynorphin. These data suggest that the relative role of opioids in hypoxic pial dilation changes with the stimulus duration.