OBJECTIVE This study investigated if the plasma vasopressin concentration increases during labour in the dog and whether the change in vasopressin correlates with that of oxytocin, 15-ketodihydro-PGF2alpha and cortisol. METHODS Five beagle dogs each delivered three to seven puppies. Blood samples were taken from a catheter inserted into the cephalic vein during labour and by venepuncture during the other periods. RESULTS Vasopressin concentration increased from 2 +/- 0 pmol L-1 (anoestrus) to 26 +/- 11 pmol L-1 at the birth of the first puppy, remained high at the birth of the second puppy and then decreased. Oxytocin increased from 63 +/- 5 pmol L-1 (anoestrus) to 166 +/- 19 pmol L-1 at the birth of the first puppy and remained elevated throughout labour. The PGF2alpha metabolite concentration increased from 0.2 +/- 0.0 nmol L-1 (anoestrus) to 66 +/- 17 nmol L-1 at the birth of the first puppy and remained elevated 1 h after the completion of parturition. The cortisol concentration increased from 49 +/- 9 nmol L-1 (anoestrus) to 242 +/- 35 nmol L-1 at the birth of the first puppy, remained high during the birth of the second puppy and then declined. CONCLUSIONS The plasma level of vasopressin was strongly correlated with that of cortisol but less with that of the PGF2alpha metabolite, and not significantly with the concentration of oxytocin. This indicates that the four hormones play different roles during labour in the dog.