Thyroid disorders are associated to a number of vascular diseases that involve processes such as platelet aggregation and vascular tone control. Since, these processes can be also affected by ATP, ADP and adenosine levels, we investigate the hydrolysis of these nucleotides in platelets from hyperthyroid, hypothyroid, and hypothyroid with hormonal replacement rats. Hyperthyroidism was induced by daily injections of L-thyroxine (T4) 25 microg/100 g body weight for 14 days. Hypothyroidism was induced by thyroidectomy and methimazole (0.05%) for 14 days. In the hormonal replacement group, hypothyroid rats were injected with T4 (5 microg/100 g body weight, i.p.) for 5 days. The AMP hydrolysis by platelets was increased 49% in hyperthyroid rats and decreased 50% in response to hypothyroidism, while the ATP and ADP hydrolysis was not altered in both groups. Besides, the T4 replacement significantly reversed the inhibition of the AMP hydrolysis observed in hypothyroid rats. Our findings indicate that the thyroid disorders affect the 5'-nucleotidase activity and consequently can alter the adenosine levels in a reversible manner in platelet fraction. Since, adenosine is able to inhibit platelet aggregation and acts as a potent vasodilator, these results can contribute to a better comprehension of the vascular events described in thyroid disorders.