1. We tested the hypothesis that the sarcoplasmic reticulum (SR) buffers (attenuates) the increase in averaged myoplasmic free [Ca2+] (Ca(im)) resulting from Ca2+ influx. 2. Fura‐2 measurements of Ca(im) were obtained in single smooth muscle cells freshly dispersed from bovine coronary artery. 3. Caffeine (5 x 10(‐3) M) elicited a transient increase in Ca(im) and depleted the SR Ca2+ store. In the continued presence of caffeine or 10(‐5) M‐ryanodine SR buffering of Ca(im) was inhibited. Subsequent exposure to high extracellular [K+] (greater than 30 mM, equimolar Na+ removal) elicited a 2‐fold more rapid and 2‐fold greater peak increase in Ca(im) than high K+ elicited when SR buffering of Ca(im) was normal. The augmented increase in Ca(im) was inhibited 35% by 10(‐5) M‐diltiazem, 65% by 2 x 10(‐4) M‐LaCl3, and 87% in Ca(2+)‐free external solution. 4. When Ca(im) buffering capacity was increased by partially depleting the SR with a transient (1 min) exposure to caffeine, subsequent exposure to 80 nM‐K+ solution increased Ca(im) almost 2‐fold more slowly than 80 mM‐K+ before depletion of Ca2+ from the SR. However, the influxing Ca2+ was sequestered by the SR and refilled it, as evident by the subsequent caffeine‐induced Ca(im) transient being identical to the first. Increasing extracellular [K+] (thus, increasing depolarization and Na+ removal) caused proportional increases in Ca(im) and the subsequent caffeine‐induced Ca(im) transients were proportionally larger, indicating a graded filling of the SR by Ca2+ influx. 5. Diltiazem (10(‐5) M) inhibited the refilling of the SR achieved by 80 mM‐K+, by 26%. Refilling was inhibited 76% by 80 mM‐K+, Ca(2+)‐free solution, indicating the fraction of refilling dependent on influx of Ca2+ through voltage‐gated Ca2+ channels, leak channels, and other influx pathways. Mild depolarization with 35 mM‐K+ (no Na+ removal) often caused no increase in Ca(im), but influx through voltage‐gated Ca2+ channels occurred because the SR Ca2+ store was refilled. Also, 10(‐5) M‐diltiazem or 10(‐6) M‐TA3090 inhibited the refilling to levels attributable only to leak influx of Ca2+. 6. All data support our hypothesis that the SR significantly attenuates the amount of Ca2+ influx that accumulates to increase Ca(im).(ABSTRACT TRUNCATED AT 400 WORDS)