1. The presence of functional alpha-adrenoceptors in freshly dispersed single smooth muscle cells from rat tail arteries was investigated by use of selective alpha-adrenoceptor agonists and antagonists. 2. Cirazoline, a selective alpha 1-adrenoceptor agonist, caused a prazosin-sensitive, rapid but transient increase in intracellular Ca2+, which was partially inhibited by the voltage-dependent Ca2+ channel blocker, nifedipine. 3. TL99, an alpha 2-adrenoceptor agonist, in the presence of prazosin, initiated a slow and sustained increase in intracellular Ca2+ which was partially inhibited by yohimbine and almost completely blocked by nifedipine. 4. In rat tail artery, the effects (dose-response and time-response curves) of cirazoline and TL99 on intracellular Ca2+ levels in freshly dispersed single smooth muscle cells were comparable with those obtained with organ bath studies of ring preparations of artery. 5. In freshly dispersed single smooth muscle cells, the time-course response curves induced by the selective alpha 1-adrenoceptor agonist, phenylephrine and the selective alpha 2-adrenoceptor agonist, UK14304, were similar to those observed with cirazoline and TL99, respectively. 6. These results indicate that: (a) functional alpha 1- and alpha 2-adrenoceptors are present in freshly dispersed single smooth muscle cells from rat tail artery and (b) alpha 1- and alpha 2-adrenoceptors are coupled to different cellular processes that lead to an increase in intracellular Ca2+.