Abstract 1. 1. Cultures of vascular smooth muscle cells accumulate extracellular breakdown products of purine and pyrimidine nucleotides that, over 9 hr, represent 60±7 and 78±17%, respectively, of the intracellular nucleotide content. 2. 2. The accumulation is stimulated during contracture with 20 mM KCl or 70 μM carbachol, consistent with the notion that both pyrimidine and purine nucleotides are involved in the energetics of smooth muscle contracture. 3. 3. Because the intracellular levels of pyrimidine and purine nucleotides remain constant, it appears likely that rates of synthesis match the rates of release. 4. 4. Ectonucleotidases are present that can degrade ATP, UTP, and CTP. High-energy nucleotides may be the primary products released.