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Synthetic cyclic growth-hormone release-inhibiting hormone (G.H.-R.I.H.) impaired platelet aggregation in each of four healthy men given 6-hour infusions. The effects lasted over 24 hours in three of them. There was no consistent change in platelet-counts during the infusions, but 18 hours after the end of the infusions there was a slight but significant increase in platelet-count. There was no change in prothrombin-time, partial thromboplastin-time, fibrinogen titres, and fibrinogen-degradation products. Incubation of G.H.-R.I.H. with blood in vitro did not affect platelet aggregation. Similar impairment of platelet function has been reported by others in baboons given linear G.H.-R.I.H. Infusions in the four healthy men studied also produced abdominal pain, dizziness, and diarrhoea in three, as have been reported in patients similarly infused. Although other side-effects or impairment of platelet-counts or bleeding-tendencies have not been reported in patients infused for up to 72 hours, caution should be exercised when using G.H.-R.I.H. over extended periods until further data on its toxicity are available.