Direct developing organisms obviate the larval intermediary from their ontogeny, hatching as miniature adults. To investigate this phenomenon, we have examined the developmental expression of arginase in the direct developing frog Eleutherodactylus coqui. An enzyme in the ornithine-urea cycle, the activation of liver arginase is necessary for the switch from ammonotelism to ureotelism which occurs when many frogs metamorphose and assume a terrestrial existence. Arginase enzyme activity is detectable at low levels in late prehatching stages of E. coqui, and increases at hatching, at which point the protein becomes detectable on Western blots. The activity increases gradually during posthatching development, reaching maximal levels at approximately the same time as yolk resorption is completed. Thyroid hormone is responsible for upregulating arginase activity during metamorphosis in Rana, but the role of thyroid hormone in direct developing frogs is unknown. A high dose (250 nM) of the thyroid hormone analogue 3,3'5triiodo-L-thyronine (T3) caused precocious induction of arginase protein and activity, showing that even in a direct developing frog, some level of responsiveness to the metamorphic trigger, thyroid hormone, has been retained. @ 1996 Wiley-Liss, Inc.