This study provides clear evidence for synergism between diuretic peptides from Locusta migratoria, Locusta-DP (a CRF-related peptide) and locustakinin (an insect myokinin). At low concentrations, the peptides act co-operatively to stimulate Malpighian tubule fluid secretion by more than the sum of their separate responses. They work via different second messengers, and their synergistic actions can be mimicked with exogenous 8-bromo-cyclic AMP replacing Locusta-DP, and/or with thapsigargin substituting for locustakinin. Additionally, 5-HT, which had been shown previously to stimulate tubule secretion via a cyclic AMP-independent mechanism, potentiated the response to Locusta-DP. Potassium is invariably the dominant cation secreted by locust tubules, but Locusta-DP, whether in the absence or presence of locustakinin, increases JNa at the expense of JK. In contrast, JNa and JK increase in parallel with JV after stimulation by locustakinin. These findings are consistent with the known actions of cyclic AMP and leucokinin-VIII on mosquito tubules. The ability to vary the relative amounts of Na+ and K+ delivered to the hindgut may be important for the maintenance of haemolymph volume and composition in recently fed locusts.