“…Concentrations of between 0.1 and 300 ng'ml (0.3-800 nmol'l) cause relaxation whereas very high concentrations of over 380 ng'ml (> 1 ,tmol'1) produce contraction (Jarman et al, 1979;Paul et al, 1982). Intravenous infusions of very large doses (4.5-83 ,tg' kg; 12-221 nmol'kg) produces radiographic dilatation of cerebral arteries (Jarman et al, 1979) and intracarotid infusion of large doses (0.1-5 Ag kg-' min-'; 0.3-13 nmol kg-' min-') increases cerebral blood flow (CBF), and reverses indomethacin-induced inhibition of hypercapnoeic vasodilatation in anaesthetised baboons (Pickard et al, 1980). These observations have led to the hypothesis that PGI2 may play a role in the control of normal CBF (Boullin, 1980;Pickard, 1981).…”