Whole blood impedance aggregometry is an important method to investigate platelet function disorders. Storage time is regarded as a main pre-analytical factor. The objective of the present study was to evaluate the influence of storage time of canine hirudin-anticoagulated blood on test results of different parameters (area under the curve (AUC), maximum aggregation and velocity) of impedance aggregometry. Hirudin-anticoagulated blood from eight healthy dogs and eight samples collected from two dogs receiving clopidogrel treatment (4 mg/kg orally once daily) were stored at room temperature for 1 day. The samples were tested at different times (0.5, 1, 2, 3, 4, 6, 8, 24 h). Samples of normal dogs were activated using 10 μmol/L adenosine diphosphate (ADP), 5 μg/mL collagen or 1 mmol/L arachidonic acid (AA), whereas in clopidogrel-treated dogs only ADP was used. Analysis of variance revealed significant differences between time points for collagen-induced aggregation in healthy dogs (maximum aggregation, velocity), AA-induced aggregation in healthy dogs (AUC, maximal aggregation) and ADP-induced aggregation in clopidogrel-treated dogs (maximum aggregation, velocity) (p < 0.05). During the first 8 h, there was a reasonable discrimination between the AUC values of ADP-induced aggregation of healthy dogs and dogs treated with clopidogrel. Changes of more than 20 % when compared to 0.5 h values were nearly exclusively detected after a storage time of 24 h. The results of our study indicate that canine hirudin-anticoagulated blood samples can be used for at least 4 h and probably up to one working day to measure whole blood aggregometry.