Abstract In the study of biological rhythms several mathematical and statistical techniques are commonly used for time series analysis (periodograms, least squares regression, spectral analysis, etc.). Nevertheless graphic methods are still widely used, due to their ability to show the structure of rhythmic patterns. Perhaps the most widely used method is the double plotted graph, which becomes necessary in certain specific cases (in determining the period of an individual ultradian component, or of a weak component masked by external influences). In order to increase the quality of graphs, for visual inspection, several methods are proposed to reduce background noise, and emphasize rhythmic components. The computer manipulation of large sets of data facilitates the application of graphic image processing methods, here described. The following procedures are exposed and discussed, in the article: averaging, linear convolution transformation, median smoothing, averaging using a rotating mask, representatio...