The way in which an animal copes with stress and anxiety may influence its propensity to display aggressive behaviour. This study aimed to measure stress coping styles in dogs undergoing an aggression test assessment for rehoming suitability. Greyhound dogs (n=34) from a canine blood bank service were recruited. Dogs were individually tested during their monthly blood donation, with salivary cortisol measurements taken 5 minutes prior to, and 20 minutes after the procedure. During the procedure, a Polar® heart rate monitor measured heart rate and R-R intervals. During the following 6 months, the dogs were tested for rehoming suitability including a test for inter-dog aggression. Greyhound dogs passed this test if they did not display any signs of aggression towards an unfamiliar dog. The pass rate for the dogs in this study was 65%. There was no significant difference in pre and post-bleeding salivary cortisol levels detected across all dogs (0.17ng/ml, 95%CI, -0.37 to 0.72, P = 0.528). However, dogs that failed the aggression test tended to have increased salivary cortisol levels after the blood collection procedure, with the difference between the pre and post-bleeding cortisol levels significantly higher in dogs that failed the aggression test compared with dogs that passed (0.97ng/ml, 95% CI, 0.11 to 1.83, P = 0.029). Mean heart rate during the procedure was greater in dogs that failed the aggression test (13.6bpm, 95% CI, 2.3 to 24.8, P = 0.018), however heart rate variability (SDNN) was greater in dogs that passed (53.5, 95% CI, 24.7 to 82.2, P = 0.001). This study demonstrates a novel method of measuring stress coping style in dogs which may correlate with aggression and could serve a broad range of future applications.