1. In a placebo-controlled, randomized dose—response study the effect of the prostaglandin analogue epoprostenol (FlolanR) on the plasma level of atrial natriuretic peptide has been investigated in 14 healthy control subjects. 2. During epoprostenol infusion, atrial natriuretic peptide increased significantly in a dose-dependent manner, while it remained unchanged during placebo infusion [2 ng min−1 kg−1: epoprostenol 13.2% versus placebo −2.9%; 4 ng min−1 kg−1: epoprostenol 13.4% versus placebo −6.1%; 8 ng min−1 kg−1: epoprostenol 40.7% versus placebo −7.8% (medians), P < 0.01 for all]. 3. Mean blood pressure and heart rate increased significantly after epoprostenol, but were unchanged during placebo infusion [8 ng min−1 kg−1; mean blood pressure: epoprostenol −5.6% versus placebo 3.2%; heart rate: epoprostenol 32.7% versus placebo 3.1% (medians), P < 0.01]. 4. It is concluded that epoprostenol given intravenously increases the plasma level of atrial natriuretic peptide. The results support the hypothesis of an interaction between the prostaglandin system and atrial natriuretic peptide.