The aim of the present study was to evaluate the impact of a shift in the light cycle and of restricted food availability on circadian rhythms of blood pressure, heart rate, and behavioral activity in freely moving rats by radiotelemetry. In rats that were fed ad lib, a shift of the light cycle by 6 h (from lights on 0700-1900 h to lights on 1300 to 0100 h) induced an immediate, but from then on gradual, shift of the circadian rhythms of blood pressure, heart rate, and behavioral activity, which took 4-5 days to fully synchronize with the new light cycle. Rats on a normal light cycle receiving feeding for 1 h only during the light period rather than ad lib feeding, showed a suppressed circadian rhythm, with the dark period values reduced to values not significantly different from those observed in the light period. In addition, during the timed feeding blood pressure, heart rate, and behavioral activity peaked to levels that were normally seen during the dark period. These data show that environmental factors such as timed feeding or changes in the light cycle have a marked influence on the circadian rhythms of blood pressure and heart rate.