A pharmacological preparation containing L-thyroxine in solution has been tested in man for thyroid substitution or suppression. Twenty-two women and 2 men, aged from 30 to 82 years, with primary hypothyroidism (10) or nodular goiter (14) were treated. Serial clinical and laboratory follow-up evaluations were performed, varying from 1 to 18 months. Five hypothyroid patients and 12 with goiter had not received any previous thyroid hormone medication. Clinical and laboratory euthyroidism was achieved in all 5 hypothyroid patients by a mean weekly L-T4 dose of 580 micrograms. Inhibited or blunted TSH response to TRH was obtained in the subjects with goiter by a mean weekly L-T4 dose of 683 micrograms. The remaining 7 patients (5 hypothyroid and 2 with nodular goiter) received first L-T4 in tablet and then L-T4 in solution. Four of these 7 patients (2 hypothyroid and 2 with nodular goiter), treated with equal doses of the two L-T4 preparations, showed lower basal and stimulated serum TSH concentration during administration of L-T4 in solution. No signs of hyperthyrodism were observed during the treatment. These data indicate that the tested preparation containing L-T4 in solution can be used for thyroid substitution or suppression, and agree with our previous determination of L-T4 content in the preparations indicating an excess of L-T4 in the solution.