Magnetic resonance imaging (MRI) may provide detailed information about anatomical changes in the brain. However, the use of diagnostic routine MRI in patients with diffuse traumatic brain injury may underestimate the severity of injury. Diffusion-weighted imaging (DWI) has been useful in the diagnosis of stroke and ischaemia. In DWI and ADC (apparent diffusion coefficient)-mapping, the random movement of hydrogen atoms mainly in the water molecule is measured. In this study we have examined the effects on brain tissue of moderate air blast waves from explosions. A specially designed shock tube was used, in which anaesthetized rats were exposed to detonation of a charge consisting of the non-electric detonating cap with a 0,5-2 g PETN explosive at a distance of 1000 mm. The exposed animals were reanaesthetized at 6, 24 and 48 hours as well as 1, 2 and 3 weeks and after the injury and examined by MRI. The T2 and DWI did not reveal any lesions in the brain of the exposed animals. ADC calculation indicated an alteration in the values over time in cortex cerebri and white matter. These changes were, however, not consistent and it is concluded that more data is needed before MRI can be used as a diagnostic tool for the evaluation of patients exposed to air blast.