Hypertrehalosemic neuropeptides from the corpora cardiaca such as the decapeptide Bld HrTH bring about a profound switch in the metabolic activity of cockroach fat body during which production of the blood sugar trehalose is stimulated while the catabolism of carbohydrate (glycolysis) is inhibited. The mechanisms of the metabolic switch are not fully understood. Incubation of isolated fat body from the cockroach Blaptica dubia with 10(-8) M Bld HrTH, for 10-60 min, stimulated glycogen breakdown and increased the content of the substrates of both the glycolytic enzyme 6-phosphofructo-1-kinase (PFK, EC 2.7.1.11) and the gluconeogenic enzyme fructose-1,6-bisphosphatase (FBPase, EC 3.1.3.11) in the tissue. The glycolytic signal fructose 2,6-bisphosphate was markedly decreased in fat body on incubation with Bld HrTH. The content of ATP was slightly reduced, while the contents of ADP and AMP were increased after incubation with the hormone. Fructose 2,6-bisphosphate is a potent activator of PFK and a strong inhibitor of FBPase purified from fat body. The activity of PFK was decreased by about 90% when the hormone-dependent changes in effectors and substrates in fat body were simulated in vitro. FBPase, in contrast, was activated about 25-fold under these conditions, suggesting the hormone to stimulate gluconeogenesis in fat body. The data support the view that fructose 2,6-bisphosphate is a pivotal intracellular messenger in the hormone-induced metabolic switch from carbohydrate degradation to trehalose production in cockroach fat body.