The aim of this study was to examine the influence of congenital portosystemic shunts (CPSS) on primary haemostasis in dogs. Bleeding time, automated platelet function analysis (PFA 100 analyser), platelet count and platelet aggregation using different methods and agonists were measured in 10 dogs with untreated CPSS and in 10 healthy, age-matched controls. Bleeding time, platelet function analysis and platelet counts did not differ significantly between groups (P>0.05). Aggregation measured using the impedance method (area under the curve) was slightly to moderately reduced with high concentrations of collagen (e.g., 5μg/mL: 2948 ± 524 vs. 3472 ± 571 AU*Min) and arachidonic acid (e.g., 1mmol/L: 1006 ± 522 vs. 1963 ± 738 AU*Min) (P<0.05), but not with adenosine diphosphate. In contrast, collagen-induced turbidimetric aggregation revealed slightly higher maximum aggregation values in dogs with CPSS. Despite the moderately altered platelet aggregation, the lack of change in global primary haemostasis screening tests indicates that dogs with CPSS do not have regularly occurring clinically relevant disorders of primary haemostasis.