The differences in second messenger control of secretion were investigated in the distal and main segments of the Malpighian tubules of the black field cricket Teleogryllus oceanicus. Secretion by the main segment was considerably increased by corpora cardiaca extract and db-cAMP. Corpora cardiaca had no effect on secreted fluid composition or intracellular elemental composition but db-cAMP increased Na(+) and Cl(-) transport, as measured by x-ray microanalysis of secreted fluids and cells. Secretion by the main segment was considerably increased by forskolin and by Sp-cAMP. Secretion in the distal segment was abolished by corpora cardiaca extract but was unaffected by db-cAMP and only slightly reduced by 8-bromo-cAMP. However, Sp-cAMP increased secretion but forskolin reduced secretion. The responses of the distal segment suggest the possibility of a multiplicity of controls through different protein kinases and adenylyl cyclases. Secretion rate in the main segment was also increased by cGMP but distal segment secretion was unaffected. Secretion from both segments was increased by 5-HT. In the main segment secretion rate was increased by Ca-ionophore and thapsigargin and decreased by verapamil. This suggests a role for Ca(2+) as a controlling second messenger. In the distal segment only Ca-ionophore had an effect on secretion rate, which was reduced. Secretion rates in both segments were decreased in Ca-free saline. In saline in which Sr(2+) replaced Ca(2+), secretion rate in the main segment was greatly increased whilst that of the distal segment was decreased, suggesting that Sr(2+) could substitute for Ca(2+) as a second messenger.