Many physiological processes of domestic animals exhibit daily rhythmicity. The goal of the present study was to investigate, in cows, the influence of different schedules of feeding on daily rhythms of blood urea and ammonia concentrations. Fifteen Italian Brown cows, from the same farm, clinically healthy and placed at the same environmental temperature and photoperiod, were used for this study. Thirty days before the experiment, all the subjects were divided into three homogeneous groups of five cows: A group was fed at 08.00, B group was fed at 16.00, both having access to food for two hours (A group 08.00 - 10.00; B group 16.00 - 18.00) and C group was fed ad libitum. Blood samples were collected at four hour intervals for 72 consecutive hours starting at 08.00 of the first day and finishing at 04.00 of the third day via intravenous cannula inserted into the jugular vein. On each serum, blood urea and ammonia concentrations were assayed. Data analysis was conducted by one-way repeated measures analysis of variance (ANOVA) and by the single cosinor method. ANOVA showed a significant effect of time on all parameters studied, in either day, with P < 0.0002 for ammonia in A group and P < 0.0001 for ammonia and P < 0.0005 for urea in B group. Cosinor analysis identified the periodic parameters and their acrophases (expressed in hours) during the three days of monitoring: urea showed nocturnal acrophases in B group, ammonia showed diurnal acrophases both in A and B groups. Our results suggest the influence of external stimuli, such as feeding time, on the rhythmic pattern of the metabolites involved in liver function and the hypothetic interaction between circadian clocks placed in the liver and the mean circadian system.