To assess a possible postmortem change in the level of cyclic AMP, the thyroids were snap-frozen at various time intervals after removal. Rats were fed a low-iodine diet (LID) with PTU for 2 weeks and a week after PTU discontinuation (PTU withdrawal). In all cases, the cyclic AMP level tended to increase as time elapsed from removing till fixing the thyroids, but in the PTU withdrawal group, the level was rapidly increased 2-fold after 5 min. In an acute experiment, the thyroids were removed under anesthesia and frozen rapidly. Intravenous administration of ovine thyrotropin (250 mU) and TRH (500 ng) brought about a rapid increase in the thyroidal level of cyclic AMP to 40% to 20% over the control level. Two weeks after PTU treatment, circulating thyrotropin was increased to a maximum of 19-fold and a further enhancement ("rebound") was observed after PTU withdrawal. PTU treatment led to an increase in the thyroidal level of cyclis AMP pre mug DNA to 60% over the control value. Following PTU withdrawal, the rise in the level of cyclic AMP returned to the normal level. However, there was no change in the concentration when it was expressed as per mg wet tissue weight. Therefore, the increase in the thyroidal concentration of cyclic AMP per mug of DNA may be due to an increase in volume of the follicular cells.