The kinetics of thyroid secretion after termination of stimulation by 100 μU/ml bovine thyroid stimulating hormone (TSH) or 5 mM cyclic AMP (cAMP) were studied using perfused canine thyroid lobes. All experiments were performed as paired comparisons, one thyroid lobe acting as a control continuing to receive infusion of the stimulator. 2.5 h after termination of TSH infusion, the secretion of thyroxine (T4), 3,5,3′-triiodothyronine (T3), and 3,3′,5′-triiodothyronine (rT3) was not significantly different from that of the control lobes. After cessation of cAMP infusion, the secretion of T4 continued unaffected for ∼40 min. Then a gradual decline in T4 release occurred. The secretion of T3 and rT3 decreased somewhat earlier leading to a transient phase with increases in the T4:T3 and T4:rT3 ratios in the thyroid effluent.