500 μg of thyrotropin releasing hormone (TRH) was injected intravenously into eight Parkinsonian patients being treated with L-DOPA and into five control patients with Parkinsonism who were not receiving L-DOPA. The mean peak serum level of TSH was 10.1 ± 3.0 (S.D.) μU/ml in the L-DOPA treated patients as compared to 28.8 ± 3.5 (S.D.) μU/ml in the control group. There was no overlap in peak serum TSH levels between patients on L-DOPA and controls. Chronic L-DOPA administration results in the inhibition of TSH secretion in response to TRH stimulation; the mechanism is unknown.