The aim of the study was to evaluate the effects of ionic and non-ionic contrast media on platelet function. In 44 patients who underwent angiography, the plasma concentration of beta-thromboglobulin (beta TG) was measured before and after ionic contrast medium (diatrizoate) administration in 22 patients and non-ionic contrast medium (iopromide) in the other 22 patients. A significant decrease in the plasma beta TG levels after intraarterial contrast medium injection occurred in both groups of patients. No significant beta TG level changes occurred in patients with normal pre-examination beta TG levels in both groups. In patients with elevated beta TG levels before arteriography, beta TG returned to normal values after contrast medium injection in both groups. There was no significant correlation between the amount of administered contrast medium and beta TG concentration after angiography. These results suggest that platelet function is not affected by either ionic or non-ionic contrast medium in patients with normal platelet activation. In patients with enhanced platelet activation, the activation became normal after contrast medium administration.