The amplitude of interrupted contractions evoked by noradrenaline or caffeine in Ca2+-free, high-K+ solutions containing EGTA or La3+ was determined in small (40-60 micron thick) bundles of guinea-pig portal anterior mesenteric vein. Interrupted contractions were produced by removing the stimulating agent as soon as the amplitude of the tension record reached its peak. The distribution of intracellular Ca2+ was determined, with electron probe X-ray microanalysis, in cryosections of preparations frozen in the relaxed state and at the peak of noradrenaline-induced contractions. Interrupted contractions of maximal or near-maximal amplitudes could be evoked every 2 min for up to 15 min in the virtual absence of extracellular Ca2+. If noradrenaline was allowed to remain in the solution throughout the period of spontaneous relaxation, a subsequent contraction could no longer be evoked in the absence of extracellular Ca2+. Interrupted contractions, similar to those evoked by noradrenaline, could also be stimulated by caffeine. The amplitude of reproducible interrupted contractions in Ca2+-free, high-K+ solution was graded with noradrenaline concentration. The ability of these smooth muscles to contract repeatedly and maximally in Ca2+-free solutions indicates the recycling of Ca2+ released from an intracellular store. The occurrence of these contractions in high-K+ (depolarizing) solutions supports the conclusion (Devine, Somlyo & Somlyo, 1972) that the release of intracellular Ca2+ is one of the mechanisms of pharmacomechanical coupling. The number of subplasmalemmal regions in which high Ca concentrations (greater than 10 mmol/kg dry wt.) were detected, with approximately 75 nm diameter electron probes, was reduced in muscles frozen at the peak of contraction, from 4.7/cell periphery in the relaxed to 1.4/cell periphery in the contracted preparations. In freeze-substituted smooth muscles, in which the membranes of the junctional sarcoplasmic reticulum could be visualized, the regions containing high Ca were identified as part of the sarcoplasmic reticulum (s.r.), indicating that the s.r. is the store from which noradrenaline and caffeine release Ca2+.