Abstract
The effect of fornix transection on the diurnal fluctua-tion in plasma corticosterone levels was investigated in adult male rhythm rats. Using the stereotaxic approach, the fornix was cut by means of a small piece of a razor blade. Plasma corticosterone levels at 8 a.m. and 6 p.m. were measured 1 week or 3 weeks after surgery. There was no diurnal variation in the plasma corticosterone concentration 1 week after fornix transection. At 3 weeks post-operatively, however, significant diurnal variations were present. The data suggest that, contrary to the assumption of other authors, the fornix is not the key structure in the mediation of extrahypothalamic influences to the medial basal hypothalamus that are essential for the diurnal rhythm in plasma corticosterone levels.