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Several letters to the editor are presented in response to articles in previous issues, including one on Henry Miller's Tropic of Cancer in the June 9, 1961 issue, another on "Too Many Drugs," in the May 26, 1961 issue, and another one on the racial segregation in the U.S.

1.1. Gel filtration and density gradinet centrifugation were used to estimate the mol. wt. of arginase from the land planarian Bipalium kewense, the land snail Helix aspersaand the laughing gull Larus atricilla. The mol. wt. of arginase from the land planarian and from the land snail was ∼240,000, while that from the gull was ∼120,000.2.2. A considertion of the known molecular weights of arginases from different species indicates there is no general correlation between the mol. wt. of arginase from a given species and that species' excretory nitrogen metabolism.

1. Ornithine δ-transaminase (l-ornithine–2-oxo acid aminotransferase, EC 2.6.1.13) and Δ1-pyrroline-5-carboxylate reductase [l-proline-NAD(P) 5-oxidoreductase, EC 1.5.1.2] were demonstrated in fat-body and flight-muscle tissues of the silkmoth Hyalophora gloveri. Arginase (l-arginine ureohydrolase, EC 3.5.3.1) is also present in these tissues. 2. Arginase, ornithine transaminase and pyrroline-carboxylate reductase are generally considered to make up the catabolic pathway for the conversion of arginine into proline. The conversion of l-[U−14C]arginine into [14C]proline by intact fat-body tissue was used to show that the enzymes in insect fat body also function in this capacity. 3. Of the three enzymes of the catabolic pathway, only arginase increased during adult development and the increase coincided with the emergence of the winged adult moth. Since proline appears to be a major substrate utilized in insect flight metabolism, the increase in arginase activity at this stage suggests a major role for arginase in proline formation.