In two separate experiments positron emission tomography (PET) was used to measure changes in regional cerebral blood flow while normal subjects performed the Stroop colour word interference test, a test of selective attention. In the first experiment performance of the Stroop task was associated with activation of right orbito-frontal and bilateral parietal structures, an unexpected result in view of previously reported findings. In addition, there were highly significant time related focal changes in rCBF. A second experiment was therefore carried out which altered the experimental parameters to replicate an earlier study. In this second experiment focal activation of the right anterior cingulate and right frontal polar cortex occurred during the Stroop task. As in the first experiment significant time effects were again apparent. To determine the functionally related brain systems during the performance of the Stroop task a correlation analysis was carried out in relation to blood flow changes induced by experimental manipulation in the right anterior cingulate. This analysis indicated the engagement of a widespread network of anterior brain regions and reciprocal inhibition of posterior brain regions during the performance of the task. The results provide evidence for the involvement of anterior right hemisphere and medial frontal structures in attentional tasks but also indicate that time effects can confound task specific activations. Furthermore subtle experimental treatment parameters, such as stimulus presentation rate, influence the degree and distribution of observed activations.