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Levels of TSH, thyroxine (T4) and triiodothyronine (T3) were measured by radioimmunoassay in plasma (TSH, T4, T3) and pituitary (TSH) of 60-day old male Long-Evans rats. Definite diurnal rhythms were demonstrated in pituitary TSH, plasma TSH and T3 in intact rats, evidenced by the statistically significant differences between zenith and nadir for pituitary TSH, plasma TSH and T3. The zenith value of pituitary TSH and the nadir values of plasma TSH, T4 and T3 were observed at the same time (2400 h) as were the nadir value of pituitary TSH and the zenith values of plasma TSH, T4, and T3 (at 1200 h). Our results indicate that the rhythmicity of pituitary TSH content is a mirror image of that of plasma TSH. Adrenalectomy not only reduced plasma corticosterone levels to almost zero, but also decreased plasma T3 and T4 levels. In adrenalectomized rats, the absolute concentration of pituitary and plasma TSH increased at 1200 h and at 2400 h and, in both cases, the difference between values at 1200 h and at 2400 h persisted. The differences in plasma TSH, T3 and T4 between 1200 h and 2400 h were also observed in sham-adrenalectomized rats. These results suggest glucocorticoids do influence the pituitary-thyroid axis, but that the rhythmicity of the pituitary-thyroid axis does not seem to depend on the rhythmicity of glucocorticoid secretion.

Locomotor activity rhythms in the hagfish Paramyxine atami were studied using the same procedures as were employed in a prior study of Eptatretus burgeri. Swimming activity under 12L: 12D (7:00-19:00 light, 19:00-7:00 dark) appeared at the beginning of the dark period and at the onset of the light period. In E. burgeri such activity occurred regularly in the first 2/3 of the dark period. P. atami exhibited a labile free-running rhythm of swimming under continuous darkness. After reversal of the light-dark cycle (7:00-19:00 dark, 19:00-7:00 light), the average time necessary to shift to the new dark period was 3.7 days, in contrast to 7.7 days which E. burgeri required. The differences in the activity patterns of the two species may be ascribed to differences in their natural habitats: P. atami resides in deep waters, and cannot be so much influenced by sunlight as E. burgeri. It may be speculated that P. atami has a relatively "delicate" circadian oscillator, and the external light used in these experiments has a relatively strong effect in regulating locomotor activity, with the result that activity shifts in a short time after reversal of the light-dark cycle, and temporary activity is evoked at the onset of the light period.