Using the Born method, based on light transmission in platelet rich plasma, the minimum effective concentration (threshold values) of several platelet agonists for inducing maximum platelet aggregation was determined in healthy dogs. The final concentrations of aggregation agonists were as follows: adenosine diphosphate (ADP) (0.5-50 micromol/L; n = 75 healthy dogs), collagen (0.5-20 mg/mL; n = 75), thrombin (0.1-5 IU/mL; n = 75), ristocetin (1-10 mg/mL; n = 10), and epinephrine (5-50 micromol/L; n = 10). Reference values for maximum aggregation with a lower limit of > 80% were achieved for agonist concentrations 25 micromol/L ADP (80-98%), > or = 10 microg/mL collagen (80-96%), and > or = 1 IU/mL thrombin (80-97%). None of the concentrations of epinephrine and ristocetin used in this study induced quantitative aggregation in the whole group of healthy dogs. We also studied platelet aggregation in 14 uraemic dogs using selected concentrations of aggregation agonists. Aggregation was significantly decreased in uraemic dogs using intermediate agonist concentrations, i.e., in the region of the threshold concentration. In contrast, maximum aggregation was increased in uraemic patients compared to reference values using low concentrations of all three agonists (ADP: 1 micromol/L, collagen: 1 microg/mL, and thrombin: 0.1, 0.2 IU/mL).