The present study was undertaken to determine the role of the hypothalamic periventricular (PV) region in the regulation of the secretory pattern of growth hormone (GH) in adult male rats. The PV regions were destroyed stereotaxi-cally by means of a modified Halasz’s knife which was lowered through the midline and rotated several times at the level of the region. In sham-operated controls, the knife was lowered to the same region but no rotation was performed. Serial blood samplings were performed at 10-min intervals for 10 h (10.00–20.00 h), via an intracardiac cannula, at 2, 4 and 6 weeks postoperatively. In 7 sham-operated controls, GH was secreted with a surge period of about 3.5 h, with peak levels of 120–170 ng/ml whole blood and basal levels of about 10 ng/ml whole blood. In 11 PV-lesioned animals, the basal levels of GH were unaltered but the surge period decreased to about 3.0 h and peak levels increased to about twice as high as in sham-operated controls. Somatostatin concentrations in the median eminence of PV-lesioned rats were significantly decreased to about 32% of that of sham-operated rats. The results suggest that, via somatostatin neurons, the PV region plays an inhibitory role in the regulation of periodicity and peak levels of GH secretory b