An investigation of the efficacy of thyrotropin-releasing hormone in amyotrophic lateral sclerosis included study of the intrathecal pharmacokinetics of this neuropeptide. Its mean elimination half-life in CSF was 0.90 hours and was monoexponential. During a 2-hour infusion, 2.75% crossed the CSF/blood-brain barrier. Infusion for 12 months with an implanted pump in three patients at a rate of 3 mg/24 hr resulted in a mean CSF steady state of 2.88, 2.42, and 2.74 micrograms/ml, respectively. Pharmacokinetic data were similar at 6 and 12 months. This is an effective system with potential uses in the treatment of other degenerative diseases.