The blood-brain barrier (BBB) regulates the amount of peripherally produced leptin reaching the brain. Knowing that the blood concentration of leptin has a circadian rhythm, we investigated whether the influx of leptin at the BBB followed the same pattern in three main sets of experiments. (a): The entry of 125I-leptin from blood to brain was measured in mice every 4 h, as indicated by the influx rate of 125I-leptin 1-10 min after an iv bolus injection. The blood concentration of endogenous leptin was measured at the same times. Blood leptin concentrations were higher at night and early morning (peak at 0800 h) and lower during the day (nadir at 1600 h). By contrast, the influx of 125I-leptin was fastest at 2000 h and slowest at 0400 h. Addition of unlabeled leptin (1 microg/mouse) significantly decreased the influx rate of 125I-leptin at all time points, indicating saturability of the transport system. The unlabeled leptin also abolished the diurnal variation of the influx of 125I-leptin. (b): The entry of 125I-leptin into spinal cord was faster than that into brain and showed a different diurnal pattern. The greatest influx occurred at 2400 h and the slowest at 0800 h. In spinal cord, unlike brain, unlabeled leptin (1 microg/mouse) neither inhibited the influx of 125I-leptin nor abolished the diurnal rhythm. (c): Higher concentrations of unlabeled leptin (5 microg/mouse) inhibited the uptake of 125I-leptin in spinal cord as well as in brain, but not in muscle. This experiment measured uptake 10 min after iv injection at 0600 h (beginning of the light cycle) and 1800 h (beginning of the dark cycle). Thus, influx of 125I-leptin into the CNS shows diurnal variation, indicating a circadian rhythm in the transport system at the BBB, saturation of the leptin transport system shows differences between the brain and spinal cord, and blood concentrations of leptin suggest that partial saturation of the transport system occurs at physiological concentrations of circulating leptin, contributing to the differing diurnal patterns in brain and spinal cord. Together, the results show that the BBB is actively involved in the neuroendocrine regulation of feeding behavior.