Besides classical effects on water balance and blood pressure, vasopressin has important functions in brain including modulation of behavioral processes and memory consolidation. In adult animals immunoactive VP in CSF exhibits a circadian pattern of release suggesting that the CSF may be a conduit for the effects of VP on diverse brain areas. To characterize the release of VP by the fetus, CSF was withdrawn continuously (1.0 ml/hr × 3 days) from the prechaismatic fossa of 5 chronically instrumented fetal sheep at 108 to 146 d gestation in 7 studies and plasma sampled intermittently (1.5 ml q 4h ×1 day) in 3 studies. Using a specific VP RIA,daily rhythms of VP in CSF were found for each fetus. Temporal profiles showed low levels of VP (12–25 pg/ml) during daylight alternating with high levels at night (30–45 pg/ml). Cycle length by frequency domain analysis was 22 to 24 hrs<. No similar fluctuations in plasma VP levels (2.1±1.0 pg/ml) were found. The amplitude of the CSF rhythms was increased (125 pg/ml) with chronic hypoxia, acute hemorrhage and in the days immediately preceding term delivery.We conclude that circadian rhythms of VP concentration in CSF but not plasma are present in the fetus during the final 25% of gestation. Changes in the amplitude and/or period of these rhythms may be important in the timing of parturition and the fetal response to suboptimal intrauterine condition.