Abstract: In the tropical western part of Paraguay, the Chaco Boreal, general acticity and grazing behavior in Bos indicus cattle were measured continuosly during the hot rainy season. The objective of this study was to determine in which way the time rhythms of these behavioral patterns were influenced by the environmental cycles (light and ambient temperature) under the conditions of various tree densities and how heat stress could be reduced by shade. 24 Nellore steers were split up in four paddocks of 4 ha size each. Three paddocks differed in tree density (10 trees ha-1, 20 trees ha-1, bush-tree stripes) and the control, was completely cleared of bush and trees. A precondition for the analysis of the circadian time structures of "general activity" and "grazing" was the long term uninterrupted measurement with the help of data logger-collars. The bio-rhythmic research on the behavioral patterns based on the time-series studies was carried out with the autocorrelation and power spectral analysis. This procedure is recognized as a very sensitive indicator of environmental stress-causing stimuli affecting the animals. It shows in which ways and to what degrees the animals` biorhythms react to alternating environmental rhythms ("Zeitgeber") such as light or ambient temperatures. In addition to that the rectal temperature of the cattle as well as their body weights were measured every week. The general activity of the cattle from the three shaded paddocks was primarily grazing motivated. Rhythms typical of cattle can be ascertained respecting the two mean curves of general activity and grazing. In this case, there were temporal well-ordered conditions which allow behavior typical of cattle. The power spectrum exhibited, extremely strong harmonic oscillations, thus, favorable performance-promoting animal-environment relations. On the paddock without trees or shade, the mean curve of general activity contrasted sharply to the grazing curve. That signifies that the general activity of the unshaded cattle was not grazing-motivated. Instead, they sought shade. From noon until sunset, their grazing activity was at a very low level. The power spectrum for grazing showed definitive disorders in the time spectrum typical for cattle. This resulted in poor daily weight gains and significantly higher body temperatures (p<0.05).