The nutritional regulation of the growth hormone liver axis has been studied in gilthead sea bream (Sparus aurata). In a first study, fingerling fish were fed three experimental diets with varying proportions of protein (34, 45 and 55%). A 60% decrease in plasma growth hormone concentration was observed with the increase of specific growth rates and dietary protein levels. An opposite response was observed in hepatic growth hormone-binding sites and plasma insulin-like growth factor-I immunoreactivity that would reflect the insensitivity of liver to growth hormone action during relatively low protein intake. In a second study, fish were fed a commercial diet (55% protein) at different feeding levels (0, 1.2, 2.7 and 5.5 g/(100 g body wt.d). An 84% decrease in plasma growth hormone concentration was observed with the increase of specific growth rates and feeding levels from 0 to 2.7 g/(100 g body wt.d). However, significantly greater growth hormone concentration was found in fish fed 5.5 g/(100 g body wt.d) when compared with fish fed 2.7 g/(100 g body wt.d). Hepatic growth hormone-binding sites and plasma insulin-like growth factor-I immunoreactivity increased with the increase of feeding levels from 0 to 2.7 g/(100 g body wt.d), but these values were lower in fish-fed 5.5 g/(100 g body wt.d) than in those fed 2.7 g/(100 g body wt.d). The physiological importance of these results remains to be clarified, though probably it is a part of the mechanism that diminishes feed utilization for growth at high feeding levels.(ABSTRACT TRUNCATED AT 250 WORDS)