1. Post-natal manifestation of plasma corticosterone rhythm was followed by individual pups until the end of 12 weeks of life. Results obtained were analysed by both visual inspection and a least squares spectrum method. The validity of the approaches used was evaluated. 2. Circadian rhythm of plasma corticosterone in sighted pups under light-dark cycle (LD) first appeared at around the third post-natal week, accompanied often with a bimodal pattern: a second peak being counterphased to the first main peak. The second peak became obscure at the fourth week, disappearing as pups matured. 3. The hormone rhythm in blinded pups whose eyes were removed within 16 h of birth was free-running under LD with a period of 24.40 +/- 0.02 h (mean +/- S.E.M.). The free-running period was stable until at least the end of the twelfth week of life. 4. The acrophase of the circadian rhythm in blind pups born and reared under LD was 180 degrees out of phase to that born and reared under DL, a reversed light-dark condition. In each case, the acrophase shifted about 8 h from that of sighted pups at the fourth week of life. 5. It is suggested that an underlying oscillator for the hormone rhythm starts to oscillate earlier than the fourth week of life and the environmental light-dark cycle prior to the fourth week determines the phase angle of the circadian rhythm in blind pups.