The objective of this study was to determine whether osmotic pump delivery of 3,5,3′-triiodo-L-thyronine (T3), thyroid inhibitors (potassium iodide, methimazole, and thiourea), or gonadal hormones (estradiol-17β and testosterone) could modify the thyroidal status of wild Atlantic cod Gadus morhua. Osmotic pumps implanted intraperitoneally delivered on average 3% (SE = 0.5%) of their initial volume per day under the experimental conditions (depth, 25 m; temperature, 5.2-7.6°C). The administration of T3 significantly increased serum levels of both T3 and L-thyroxine (T4). Treatment with either methimazole or thiourea significantly depressed serum T3 levels to about 55% of control values, whereas treatment with testosterone significantly reduced T4 levels to about 35%. We conclude that the delivery of thyroid or antithyroid agents by osmotic pumps is an effective means for creating experimental hyper- or hypothyroidism in wild Atlantic cod.