Recent data demonstrated that somatostatin (SRIH) analogues octreotide is effective in Graves' ophthalmopathy (GO), but their mechanism of action in GO is still unclear. In this study we investigated the expression of SRIH receptor (sst1-5) genes and the effect of octreotide treatment on primary cultures of fibroblasts established from retroorbital tissue of GO patients and of control subjects.Retro-orbital connective tissue was obtained from 10 patients with GO and from 6 control subjects undergoing eye surgery. Fibroblasts were established in MEM with 5-10% FCS. The expression of sst1-5 genes was studied by RT-PCR using specific primers and GAPDH as internal control. Cells were treated with octreotide (10-8M, 10-9M) for 48-72-96 h to evaluate cell growth by MTT, cAMP accumulation by RIA and apoptosis by TUNEL techniques.All primary cultures expressed one or more ssts genes that have a high affinity for the two analogues (class 1 sst). The sst2 transcript was found in 9, sst3 in 5 and sst5 in 8 out of 10 GO cell cultures. sst2 was detected in all 6, and sst3 in 4 of the 6 control cell cultures. Octreotide (10-6 and 10-7M) significantly inhibited cell growth (p<0,01-0,05), significantly decreased forskolin-induced-cAMP accumulation, and determined apoptosis (10-18%).Our study demonstrated that sst transcripts are expressed and functional in cultured retroorbital fibroblasts. The presence of class 1 sst in GO tissue and the inhibition exerted by octreotide on retroorbital cell growth and activity in vitro may account for the effects of SRIH analogue administration in vivo in GO.