The calmodulin and calcium dependence of human adenylate cyclase from the second part of the duodenum was assessed in washed particulate preparations of biopsy specimens by investigating (a) the concentration dependent effects of free [Ca2+] on enzyme activity, (b) the effects of exogenous calmodulin on enzyme activity in ethylene glycol bis (b-aminoethyl ether)N,N'-tetra-acetic acid (EGTA) washed particulate preparations, and (c) the effects of calmodulin antagonists on enzyme activity. Both basal (IC50 = 193.75 (57.5) nmol/l (mean (SEM)) and NaF stimulated (IC50 = 188.0 (44.0) nmol/l) adenylate cyclase activity was strongly inhibited by free [Ca2+] greater than 90 nmol/l. Free [Ca2+] less than 90 nmol/l had no effect on adenylate cyclase activity. NaF stimulated adenylate cyclase activity was inhibited by 50% at 2.5 mmol/l EGTA. This inhibition could not be reversed by free Ca2+. The addition of exogenous calmodulin to EGTA (5 mmol/l) washed particulate preparations failed to stimulate adenylate cyclase activity. Trifluoperazine and N-(8-aminohexyl)-5-IODO-1-naphthalene-sulphonamide (IODO 8) did not significantly inhibit basal and NaF stimulated adenylate cyclase activity when measured at concentrations of up to 100 mumol/l. These results suggest that human duodenal adenylate cyclase activity is calmodulin independent but is affected by changes in free [Ca2+].