Conflicting data have been reported about the impaired sensitivity to the inhibitory effect of prostacyclin (PGI2) in platelets from patients with diabetes. In the present paper we investigated binding of and sensitivity to PGI2 of platelets from insulin dependent (IDDM) (n = 9), non insulin dependent (NIDDM) (n = 8) diabetics and two groups of ten healthy subjects of equivalent age in relation to platelet lipidic content. Platelet sensitivity to PGI2 (PGI2 IC50) was found not significantly changed in diabetics as compared to controls; similarly, no significant differences of the number of high affinity receptors for PGI2 in platelets from patients with IDDM and NIDDM were observed. Platelet sensitivity to PGI2 and PGI2 receptors were found to be significantly related to platelet cholesterol content (r = 0.89, p less than 0.001 and r = -0.80, p less than 0.001 respectively). In conclusion platelet PGI2 receptor changes are not detectable in diabetics in good metabolic control, but could take place when platelet lipid composition is altered.