Thymidine kinase activity of the chick pineal gland.

Pineal thymidine kinase activity of 1-week-old chicks in situ varied significantly throughout the day. However, the circadian rhythm of thymidine incorporation seen with cultured chick pineal glands was not accompanied by variations in level of thymidine kinase activity in vitro. Thus the circadian rhythm in rate of cumulative incorporation of thymidine by cultured chick pineal glands is not determined by a rhythm in rate of the first reaction of the complex series of reactions by which thymidine is incorporated into DNA.