Exposure to the extrauterine environment is associated with marked stimulation of the hypothalamic-pituitary-thyroid system after which, pituitary-thyroid equilibrium must be reestablished. This marked endogenous perturbation offers the opportunity to study the manner in which the pituitary-thyroid axis is reequilibrated. T4, T3 and TSH concentrations have been measured by RIA in sera from 440 healthy newborn infants, whose ages ranged from birth to 236 h. Results were analyzed by nonlinear curve-fitting procedures to assess the changes in mean hormone concentrations with age (t). Equations have been derived by Danziger and Elmergreen to allow assessment of oscillatory behavior during hormone equilibration. Applying these equations to the present data, we observed the presence of an oscillatory cosine term in the equation for each hormone. This indicates significant oscillations in serum T4, T3, and TSH concentrations during the first 5 days of life. The period of the oscillations approximates 16 h. The oscillations in T4, lag 1/2 to 3/4 cycle behind TSH; T3 lags behind T4. Thus, disturbances in hypothalamic-pituitary-thyroid equilibrium seem to be followed by periodic oscillations in hormone concentrations; these oscillations decrease in amplitude as the negative feedback system establishes new equilibrium conditions.