The effects of calcium concentrations in the diets of laying hens on plasma concentrations of progesterone, oestradiol and testosterone and on egg production were examined. Forty-five Isabrown hens of the same age were used and were housed together from 16 weeks of age. Up to the 20th week of age the birds were fed a pullet ration. Subsequently, the hens were divided into three groups comprising 15 animals each. The calcium concentrations in the diets fed to these groups were 1.02%, 3.11% and 4.5%. The respective mean values of plasma progesterone were 293+/-93, 425+/-240 and 475+/-215 pg/ml; significant differences were present between the first and second group and the first and third group. No significant difference between the second and third group was observed. Similar results were obtained for testosterone concentrations. There were no significant differences in diol-17beta between the three groups. No correlation between egg production and the plasma concentrations of individual hormones was found.