The present study investigated the effects of diabetes and high glucose on GHRH receptor (GHRH-R) mRNA and protein levels, in the pituitary of diabetic rats, 2, 21 and 60 days post-streptozotocin (STZ) administration. Two days post-STZ, the 2.5-kb GHRH-R mRNA transcript was increased. Twenty-one days post-STZ, both the 2.5- and 4-kb transcripts, and a 72-kDa 125I-GHRH-GHRH-R complex were elevated. Sixty days post-STZ, the 4-kb transcript remained increased and the 45-kDa 125I-GHRH-GHRH-R complex (functional receptor) was decreased. Hypothalamic GHRH mRNA and serum total IGF-1 levels were reduced at all three time points. To better understand the role of high glucose on GHRH-R regulation, time-course effects of 33 mM compared to 6 mM D-glucose (D-G) were examined in cultured anterior pituitary cells from 2-month-old healthy rats. Membrane lipoperoxidation was present in 33 mM D-G, and GHRH-R mRNA levels were diminished after 24 h, Fluo-GHRH internalization was marginal after 16-24 h and GHRH-induced cAMP levels were decreased after 24 and 48 h. Altogether, these results indicate that the increase of the 2.5-kb GHRH-R mRNA transcript in vivo could be a consequence of a decrease of hypothalamic GHRH mRNA levels in STZ rats. Since it does not affect primarily functional GHRH-R levels, the initial diminution of circulating IGF-1 levels could result from a decreased GHRH-R stimulation by GHRH. Thus, the effect of glucotoxicity would be related to a decrease of functional GHRH-R protein, as observed in rats 60 days post-STZ and in cultured pituitary cells from healthy rats, exposed to a high-glucose environment.