SUMMARY: 1. A shuttle-box technique (unconditioned stimulus—electric shock; conditioned stimulus—buzzer) has been used to study conditioned avoidance learning in rats thyroidectomized on the day of birth and during adult life. The effects of medication with thyroid hormone have also been examined. 2. Both classes of thyroidectomized rat made significantly more errors than did normal litter-mates, an effect which was more marked following neonatal thyroidectomy than in animals thyroidectomized when adult. As a result of thyroidectomy the latencies of the response to both conditioned and unconditioned stimuli increased but rate of performance and the number of inter-trial crossings was reduced. 3. Replacement therapy restored all these measures in the direction of normal. In most instances the differences in performance between normal and thyroidectomized rats of both classes was eliminated by medication, the behaviour of medicated thyroidectomized rats differing significantly from that of unmedicated animals and but little from the controls. 4. The relationship of these findings to the developmental and metabolic functions of the thyroid and to earlier behavioural studies on the significance of the age of thyroidectomy and medication are discussed.