A technique for studying in vivo the production rate and turnover rate constant of mouse brain M-receptors was established. A single injection of 25 mg / kg of Benzilylcholine Mustard to living mice resulted in 90 % irreversible block of brain M-receptors. The time course of the receptor density was then monitored by ³H-QNB binding assay and the production rate and turnover rate constant were calculated from the time course curve with a computer program. It was found that in normal mice the turnover rate constant was about 0.035 h^-1 (half-life was about 20 h) and the production rate was 30-42 fmol / (h · mg protein). Parallel experiments revealed a significant slow down of the turnover of brain M-receptors in hypothyroid mice (turnover rate constant was 0.0257±0.0012 h^-1 in hypothyroid vs. 0.0356 ± 0.0021 h^-1 in normal) while the production rate was not changed significantly. The results suggest that thyroid hormones have a regulatory action on the turnover of brain M-receptors and the elevation of brain M-receptor density together with slow down of the turnover of brain M-receptors is probably one of the important mechanisms relevant to the brain dysfunction in hypothyroidism.