Duodenal absorption of calcium, renal 25-hydroxycholecalciferol-1-hydroxylase activity, percentage bone ash, growth rate and plasma levels of Ca and P were all measured in chicks which had been treated with the synthetic glucocorticoid, betamethasone, at the therapeutic dose level of 25 microgram/kg daily. It was found that treatment for 2-3 weeks resulted in a significant reduction in Ca absorption rate, renal 1-hydroxylase activity and growth rate but an increase in bone ash and plasma Ca. If betamethasone was administered for 10 days and then stopped 4-13 days before these parameters were measured, a compensatory increase was found in 1-hydroxylase activity while the Ca absorption rate attained control values in all but the 4 day group in which it was still significantly depressed. It was concluded that a therapeutic dose level of betamethasone depresses both duodenal Ca absorption and renal 1-hydroxylase activity although there may be no direct causal relationship between these two parameters. Recovery was complete in 5-7 days after cessation of treatment.