Diurnal and episodic variations of plasma hydrocortisone concentrations in horses

Using a specific high-performance liquid chromatographic technique, plasma hydrocortisone values were measured hourly in 6 horses and every 10 minutes in 4 horses over 24 hours. Both circadian and episodic variation was observed. The mean plasma hydrocortisone concentration was a maximum of 58.8 ± 9.54 ng/ml at 9.19 ± 0.59 hr and a minimum of 27.85 ± 6.85 g/ml at 21.19 ± 0.59 hr. The number of episodes of secretion was 10.0 ± 1.41; the mean amplitude and duration of a peak were 26.21 ± 3.71 ng/ml and 105.25 ± 21.24 min respectively.