Development of diurnal rhythm in some metabolic parameters in foals. 
1.  The development of diurnal rhythm activity of FDPA, AspAT and A1AT and in levels of cortisol, T3 and T4 was observed in the blood serum of six foals.  2.  The studies began when a foal was 7 days old and were repeated every month until foals reached 1 year of age.  Blood samples were taken every 4 hr for one day each month.  3.  As a control group four barren mares were used, kept and examined in the same conditions.  4.  In mature mares, diurnal rhythms in activity of A1AT (acrophase at 2200 hr), AspAt (2400 hr) and cortisol (0630 hr) but in T3 only in summer months (acrophase at 0100 hr) were observed.  5.  During the first 6 months of foal life, significantly higher mean levels of FDPA, A1AT, T3 and T4 than in control mares were found.  6.  The cortisol level in foals was half as much as that of mature mares throughout the year.  7.  In foals the diurnal rhythm in A1AT activity occurred in the 5th month and in AspAt--in the 12th month (acrophase at 2400 hr), but in cortisol levels it was developed already in the second month of foal life (acrophase at 0830 hr).