Thyroid hormones (THs), including triiodothyronine (T3) and tetraiodothyronine (T4), are recognized as key metabolic hormones of the body. THs are essential for normal maturation and function of the mammalian central nervous system (CNS) and its deficiency, during a critical period of development, profoundly affects cognitive function. Sodiumpotassium adenosine 5-triphosphatase (Na+, K+-ATPase) is a crucial enzyme responsible for the active transport of sodium and potassium ions in the CNS necessary to maintain the ionic gradient for neuronal excitability. Studies suggest that Na+, K+-ATPase might play a role on memory formation. Moreover, THs were proposed to stimulate Na+, K+-ATPase activity in the heart of some species. In this work we investigated the effect of a chronic administration of L-thyroxine (LT4) or propylthiouracil (PTU), an antithyroid drug, on some behavioral paradigms: inhibitory avoidance task, open field task, plus maze and Y-maze, and on the activity of Na+, K+-ATPase in the rat parietal cortex and hippocampus. By using treatments which have shown to induce alterations in THs levels similar to those found in hyperthyroid and hypothyroid patients, we aimed to understand the effect of an altered hyperthyroid and hypothyroid state on learning and memory and on the activity of Na+, K+-ATPase. Our results showed that a hyper and hypothyroid state can alter animal behavior and they also might indicate an effect of THs on learning and memory.