Intracellular [Ca2+] within the smooth muscle of blood vessels is an important determinant of cell contraction and, therefore, vascular resistance. Ca2+-imaging of smooth muscle cells in the walls of intact retinal arterioles reveals both brief, spatially localised events resembling Ca2+-sparks, and prolonged Ca2+-oscillations (McGeown et al, 2004). Here we report results from experiments testing the role of ryanodine-receptor (RyR) linked Ca2+-release in these events. Sprague Dawley rats (200-300g) were anaesthetised and killed by cervical dislocation. Arterioles were dispersed from retinae by trituration, and incubated with 10μM Fluo-4AM. Changes in [Ca2+]i were confocally imaged in smooth muscle cell arrays (9-17 cells). Arterioles were superfused with saline at 37oC and linescanned at 500 scans s-1. Fluorescence data (F) were extracted, background-corrected and normalized to resting fluorescence (F0). Changes in [Ca2+]i were analysed in terms of their amplitude (ΔF/F0), full duration at half maximal fluorescence rise (FDHM), rise time to peak fluorescence, and full width at half maximal fluorescence (FWHM). The average speed of propagation was calculated using the average slope of the half-maximal fluorescence contour. Differences in means were assessed statistically using paired or unpaired Student’s t-tests. In 26 cells from 6 arterioles, the frequency of brief, spark-like events was decreased from 0.54±0.07 s-1 (mean±SEM) under control conditions to 0.07±0.04 s-1 during 30s superfusion with tetracaine (100μM; P<0.0001). Although tetracaine abolished spark activity in over 70% of cells, it persisted in 7 cells. Mean spark amplitude was not altered in these cases but their duration (FDHM) was increased from 41.0±5.3 ms to 175.4±21.6 ms (P<0.00001). This prolongation largely reflected slowing of the rising phase, total rise time being increased from a control value of 29.9±2.8 ms (n=44 events) to 117.6±17.6 ms (n=36 events) during tetracaine superfusion (P<0.0001). The speed of spread of sparks also fell from a control value of 71.2±11.6 μms-1, to 15.8±2.0 μms-1 for events initiated at the same sites in the presence of tetracaine (P<0.0001), while FWHM fell from 1.59±0.20 μm to 0.99±0.11 μm (P<0.02). These results suggest that RyR-gated Ca2+-release channels are important both for the initiation and propagation of spontaneous transients in retinal arteriolar vascular smooth muscle.