1. A mathematical model has been constructed of human thyroid hormone regulation by the anterior pituitary gland, which takes account of most of the currently available experimental data. 2. Successful simulation of data on the stimulation of thyrotrophin (TSH) secretion by thyrotrophin releasing hormone (TRH) was achieved assuming that the TSH secretion rate is proportional to the logarithm of the concurrent blood TRH level. 3. Data on the regulation of triiodothyronine (T3) secretion by TSH and the inhibition of TSH secretion by thyroid hormones in contrast could not be simulated on the assumption of instantaneous proportional responses. A mixture of proportional and integral control--the latter taking account of the past history of plasma levels of the regulatory hormone--appeared to be operating at both levels. 4. The pituitary gland appears to be more sensitive to a given fractional change in TRH secretion rate than to the same fractional change in T3 plasma concentration.