All tissues of the embryonic saltwater crocodile (Crocodylus porosus) gradually increased in weight during development except for lung tissue, which had a peak weight of 1.09 g at day 67, thereafter decreasing in weight. The brain was a relatively large organ. Deiodinase activities in liver, kidney, lung, heart, gut, and brain from day 29 to day 77 of development of the saltwater crocodile were investigated. High-K(m) reverse triiodothyronine (rT(3)) outer ring deiodination (ORD) activity was present in all tissues except the brain. Activity ranged from 559 +/- 51.3 pmol rT(3) deiodinated/mg protein/min in the liver at day 77 to below 10 pmol rT(3) deiodinated/mg protein/min in gut, lung, and heart tissue. rT(3) ORD increased during development in the liver and kidney but decreased in the gut and lung. Activity in the heart was very low (less than 2 pmol rT(3) deiodinated/mg protein/min) and did not change during development. Low-K(m) thyroxine (T(4)) ORD in liver and kidney tissue had peaks of activity around day 49 of incubation (0.52 and 0.09 fmol T(4) deiodinated/mg protein/min, respectively). After day 49, T(4) ORD activity in these tissues decreased. T(4) ORD activity in gut, lung, and heart was very low (less than 0.04 fmol T(4) deiodinated/mg protein/min), with activity in lung increasing slightly during the rest of development. T(4) ORD activity in the brain increased toward day 77 (0.14 +/- 0.03 fmol T(4) deiodinated/mg protein/min), illustrating its importance in local triiodothyronine (T(3)) production during brain development. T(3) inner ring deiodination activity was present only in the embryonic liver and peaked at day 49 (10.1 fmol T(3) deiodinated/mg protein/min), after which activity decreased.