WHEN commenting generally on secondary metabolic phenomena and on the particular example of the synthesis of 6-methylsalicylic acid (6-MS) in Penicillium urticae Bainier (≡P. patulum Bainier, etc.), we referred1 to unpublished evidence suggesting that 6-MS could exert feedback inhibition on its own synthesis. Recently, Arihood and Light have described2 investigations of this problem, and have reported that the addition of 6-MS, or of certain related compounds, inhibits the incorporation into 6-MS of carbon-14 from exogenous 1-14C-acetate in intact mycelium. Our own work, however, revealed unsuspected complexities in this type of experiment. These have been summarized elsewhere3, but the publication of Arihood and Light prompts us to record some of our evidence in more detail. The basic experimental techniques and the general dynamics of 6-MS synthesis have already been described4.