||No detailed comparative data are available on the hormonal parameters of horses suffering from a number of diseases. The aim of our study was to measure concentrations of cortisol, adrenocorticotropic hormone (ACTH), serotonin, adrenaline and noradrenaline in horses with various diseases and following surgery, to assess the response of the HPA axis and adrenal medulla. Blood samples were obtained from six groups of horses comprising a total of 119 animals as follows: laminitis, acute abdominal syndrome (AAS), castration surgery, acute diseases, chronic diseases and healthy controls. Serum hormonal concentrations were determined for each group for comparison. Statistically significant differences between all groups and controls were found for cortisol, ACTH (except for castration), serotonin and adrenaline concentrations but only in horses with laminitis and AAS for noradrenaline. No statistically significant differences were found between males and females. The largest changes in the pituitary–adrenal axis activity occurred mainly in acute diseases, laminitis and in the AAS group.