. Content-based search in image databases are generally done by matching extracted features. A complementary approach could be the use of intensity based similarity measures, providing global information. Recent image registration procedures involve the use of robust similarity measures such as the Mutual Information or the Correlation Ratio. Although the statistics theory or signal theory show there are a lot of measures potentially suitable for this task, the practitioners of images processing limit themselves to the use of one or two measures. We propose in this paper to investigate some other similarity measures for image processing, based on Mutual Information, and applied to image registration. In this preliminary study, we rst present the implicit assumptions made when the notion of similarity between images is considered as related to their intensity distribution. Then, we perform some experiments to provide a rst rough evaluation of the suitability of the measures. 1 Introdu..