Bornaviruses are considered to be the causative agent of proventricular dilatation disease (PDD) in psittacine birds. In order to detect haematological and blood chemistry changes during the development of PDD and a possible correlation with clinical signs and the virological status, six African grey parrots (Psittacus erithacus) were experimentally infected with parrot bornavirus 4 (PaBV-4) by subcutaneous route. All six parrots developed clinical signs of varying extent and successful infection was confirmed in all the birds by seroconversion or detection of RNA of the PaBV-4 infection strain. Based on population-based and intra-individual reference ranges established during 12 months prior to experimental infection, only minor haematological changes were detected in individual birds after infection. Changes in blood chemistry were restricted to aspartate aminotransferase, creatine kinase, total protein, glucose and uric acid. Plasma protein electrophoresis revealed marked changes starting 10 weeks post infection characterized by an increase in the γ-globulin fraction and a gradual decrease to normal values during weeks 22-34. Indications of an acute-phase reaction at the initial stages of infection were not detected. While three birds suffered from clinical signs of PDD, which included weight loss and neurological disorders and died before development of haematological and plasma protein changes, recovery of clinical disease was paralleled in the remaining birds by an increase in γ-globulins and bornavirus-specific antibody titres.