The free segments of the Malpighian tubules of Tenebrio molitor larvae, and the rectal complexes, have been studied as isolated preparations. Fluid secretion by the free tubules is stimulated by extracts of corpora cardiaca, and this stimulation is reflected in a marked increase in intracellular cyclic AMP levels. The tubules of hydrated mealworms secrete at higher basal rates and their corpora cardiaca show less diuretic activity than those of “dry” mealworms. There are no regional differences in tubular secretion rates, but some of the Na + is replaced by additional K + in the lower parts of the tubules, giving a high final K + concentration in the tubular fluid. Simple isolated preparations were made of rectal complexes, with fluid being collected from individual tubules as they emerge from the common duct. This fluid was isosmotic with the bathing Ringer and high in K +. The rate of fluid production and the osmolality were unaffected when corpora cardiaca extracts were added to the bathing Ringer. Because fluid leaving the complex rapidly reaches osmotic equilibrium with the haemolymph, the free segments of the Malpighian tubules function essentially independently of the rectal complex. The hormonal stimulation of fluid secretion may be important in hydrating the dry food eaten by tenebrionid beetle larvae, the tubule fluid passing anteriorly into the midgut.