The postmortal decay of acetylcholine (Ach) was studied in the cervical spinal cords of rats in conditions of hyper- and hypothyroidism. The modifications of thyroid function were achieved either by chronic (20–25 days) administration of /-thyroxine or of methimazole. The basal metabolic rate and plasma T4 concentration were measured to estimate the degree of modification of thyroid activity. The levels of Ach at the start of postmortal decay were evaluated by extrapolation to time 0 of the curves of the postmortal decay of Ach and the levels of Ach at stabilization were estimated from the means of all the measures made at lapses of time over 100–200 s from death. In low and high hypothyroidism a reduction (53 and 72%, respectively) of the levels of Ach was found. A similar effect was found in hyperthyroidism: a 73 and 63% reduction of Ach levels in high and low hyperthyroidism, respectively. The level of Ach at stabilization of the postmortal decay increased only in hyperthyroid rats. The process by which Ach is destroyed is not modified in hyper- or hypothyroid