Fura2 - fluorescence was utilized to test for the effect of extracellular nucleotides on intracellular calcium concentration of subconfluent Madin-Darby Canine Kidney (MDCK)-cells. Extracellular ATP (10 mumol/l) and UTP (10 mumol/l) lead to rapid (within seconds), sustained, and fully reversible enhancement of intracellular calcium concentration from 138 +/- 9 nmol/l (n = 27), to 1561 +/- 260 nmol/l (n = 10) and 3435 +/- 949 nmol/l (n = 5), respectively. Half maximal effects are observed at some 1 mumol/l. In the absence of extracellular calcium the effect of ATP is transient, pointing to release of intracellular calcium. The sustained effect in the presence of extracellular calcium indicates that the nucleotides in addition recruit calcium from extracellular space.