The effects of octreotide on transplanted azaserine-induced pancreatic acinar tumours were investigated in the rat. When tumours became palpable, rats were treated either with octreotide (40 micrograms/kg per day, by infusion) or NaCl 0.9% (controls) for 14 days. Tumours were then analysed for their size, composition and somatostatin receptors. Octreotide induced a 80% reduction in tumour growth rate during the first 2 days of treatment. This rate was less marked from day 4 to day 15. The tumour weight, protein, DNA, RNA and enzyme content were reduced in parallel by 50 to 60%. A homogeneous distribution density and a high affinity of somatostatin receptors were found by receptor autoradiography and in vitro binding assays in tumours of both groups. These findings indicate that octreotide reduces the growth rate of the transplanted pancreatic acinar tumour and may exert its inhibitory effect directly via specific somatostatin receptors on tumour cells.