Para-aminohippuric acid (PAH, 0.2 and 1 mmol l(-)(1)) had no effect on the basal fluid secretion rate (FSR) of isolated Malpighian tubules of Drosophila melanogaster Meig. and did not affect stimulation of the FSR induced by adenosine 3',5'-monophosphate (cAMP). Phenol Red (phenolsulphonphthalein, PSP; 0.5 and 1 mmol l(-)(1)) slowed the FSR and abolished stimulation of the FSR by cAMP. Diodrast (1 mmol l(-)(1)) slightly, but significantly, reduced the FSR and greatly reduced the stimulation of the FSR normally provoked by cAMP and by the 3',5'-monophosphates of guanosine (cGMP), inosine (cIMP) and uridine (cUMP). However, stimulation of the FSR by the 3', 5'-monophosphate of cytidine (cCMP) was little affected by diodrast. Probenecid (0.2 or 1 mmol l(-)(1)) consistently stimulated the FSR, on average by approximately 25 %, but did not markedly inhibit the subsequent stimulation of the FSR by cAMP, cGMP or cIMP. However, the FSR of tubules stimulated by cGMP was temporarily lowered by probenecid. Quinacrine (0.1 mmol l(-)(1)) slowed basal FSR by an average of approximately 30 %, but subsequent stimulation of the FSR by cAMP was not noticeably affected. Both 0.1 mmol l(-)(1) cAMP and 1 mmol l(-)(1) probenecid stimulated adenylate cyclase activity in extracts of Malpighian tubules, but cIMP, cGMP, cUMP and diodrast were without effect in this regard. Uptake of radioactivity from a solution containing 500 nmol l(-)(1) [(3)H]cAMP and 9.5 micromol l(-)(1) cAMP was reduced by more than 90 % by 1 mmol l(-)(1) PSP, by approximately 40 % by 0.2 mmol l(-)(1) probenecid, by 36 % by 1 mmol l(-)(1) diodrast and by 30 % by 1 mmol l(-)(1) PAH. Neither 0.01 mmol l(-)(1) ouabain nor 0.1 mmol l(-)(1) quinacrine affected the uptake of [(3)H]cAMP by the Malpighian tubules. Fluid secreted by isolated Malpighian tubules of Drosophila melanogaster contains a factor that stimulated the FSR on average by approximately 50 %. The presence in the secreted fluid of cGMP at a concentration of 8.3 micromol l(-)(1) did not explain the stimulatory effect on FSR. These results support the existence of a carrier-mediated uptake of cyclic nucleotides into the Malpighian tubules of Drosophila melanogaster, possibly involving a multispecific transporter.