Effects of co-dergocrine mesylate (Hydergine), a drug widely used for the therapy of cerebral vascular disease on local platelet accumulation in the carotid artery region was studied by means of the platelet uptake ratio (PUR) and on the systemic platelet-vascular wall interaction as calculated from platelet half-life were investigated. A placebo controlled, double blind, randomised protocol was used, 18 patients were treated with co-dergocrine and compared to placebo (n = 18). Co-dergocrine treatment resulted in a significant decrease in platelet deposition, PUR decreased from 1.28 +/- 0.05 before treatment to 1.25 +/- 0.06 on day 5 of therapy with a statistically significant (p less than 0.001) in the paired comparison. In the control group the corresponding changes from 1.29 +/- 0.04 before to 1.28 +/- 0.04 did not show a p-value of less than 0.05 in paired comparison. Platelet half-life (72 +/- 11 before vs. 76 +/- 11 hours after 5 days of co-dergocrine treatment) showed a statistically significant (p less than 0.001) prolongation, whereas in the placebo group no relevant change of T/2 was observed (71 +/- 10 before vs. 72 +/- 10 hours on day 5, p greater than 0.10). No relevant effects on ADP-induced platelet aggregation, platelet-release reaction, platelet aggregate ratio, TXB2 plasma levels and thrombin-induced MDA-formation could be detected. These results indicate that co-dergocrine decreased in-vivo platelet residence time to atherosclerotic lesions of the carotid artery. Co-dergocrine may thereby be of benefit in prevention of mural thrombus formation and prevention of transient ischemic attacks, but also of atherosclerosis in man.