The effects of 5-hydroxytryptamine (5-HT; serotonin) and dopamine (DA) on tissue carbohydrate metabolism and haemolymph glucose levels in the freshwater prawn, Macrobrachium malcolmsonii, were investigated. Injection of 5-HT and DA produced hyperglycaemia in a dose-dependent and time-dependent manner, with DA being more effective than 5-HT. Interestingly, 5-HT and DA induced hyperglycaemia only in intact prawns but not in bilaterally eyestalk-ablated individuals. Total carbohydrate (TCHO) and glycogen levels decreased and phosphorylase activity increased in the hepatopancreas and muscle of intact prawns after being injected with 5-HT or DA. However, bilateral eyestalk ablation decreased haemolymph glucose and tissue phosphorylase activity and increased TCHO and glycogen levels of the hepatopancreas and muscle. Injection of 5-HT or DA did not cause significant changes in these variables in eyestalk-ablated prawns. It is hypothesized that 5-HT and DA induce hyperglycaemia in prawns by stimulating the release of crustacean hyperglycaemic hormone (CHH) from the X-organ sinus gland (XO-SG) complex located in the eyestalk. J. Exp. Zool. 303A:448–455, 2005. © 2005 Wiley-Liss, Inc.