To examine the physiological significance of plasma ghrelin in generating pulsatile growth hormone (GH) secretion in rats, plasma GH and ghrelin levels were determined in freely moving male rats. Plasma GH was pulsatilely secreted as reported previously. Plasma ghrelin levels were measured by both N-RIA recognizing the active form of ghrelin and C-RIA determining total amount of ghrelin. Mean +/- SE plasma ghrelin levels determined by N-RIA and C-RIA were 21.6 +/- 8.5 and 315.5 +/- 67.5 pM, respectively, during peak periods when plasma GH levels were greater than 100 ng / ml. During trough periods when plasma GH levels were less than 10 ng / ml, they were 16.5 +/- 4.5 and 342.1 +/- 29.8 pM, respectively. There were no significant differences in plasma ghrelin levels between two periods. Next, effect of a GH secretagogue antagonist, [D-Lys-3]-GHRP-6, on plasma GH profiles was examined. There were no significant differences in both peak GH levels and area under the curves of GH (AUCs) between [D-Lys-3]-GHRP-6-treated and control rats. These findings suggest circulating ghrelin in peripheral blood does not play a role in generating pulsatile GH secretion in freely moving male rats.