Conscious, adult male, sham-operated (control), and pinealectomized (Pect) rats with chronic jugular cannulae were bled during the day or night. Daytime samples were obtained from rats housed singly in small cages, while the night samples were obtained from animals on a reversed light-dark cycle kept in a larger isolation cubicle. During the day, controls showed frequent secretory bursts of GH release, as evidenced by dramatic elevations of plasma GH with a periodicity of 2.3 h. In the Pect rats, there was a more variable GH release, with some animals showing spikes similar in amplitude to those of controls and others showing no spikes. There were significantly fewer GH values in excess of 150 ng/ml after Pect, and GH release over the total period of sampling, calculated from the area under the plasma GH curve, was significantly reduced. During the night, plasma GH spikes of controls were significantly reduced in frequency and amplitude, but these were not further significantly reduced by Pect. Under our conditions, there was a diurnal change in GH release with reduced release at night. Pinealectomy significantly reduced GH release during the day but did not significantly further reduce GH release at night.