Intracellular [Ca²⁺] ([Ca²⁺]_i) is an important regulator of function in smooth muscle cells. When microvascular smooth muscle (MVSM) cells are imaged in situ in intact retinal arterioles, a variety of spontaneous [Ca²⁺]_i transients are seen (Curtis et al. 2003). These have now been characterised further. Sprague Dawley rats (200300g) were anaesthetised and killed by cervical dislocation. Arterioles were mechanically dispersed from retinae, incubated with 10µM Fluo-4AM, and then superfused at 37^oC. They were linescanned using a confocal scanning laser microscope (Biorad, MR/A1, 500 scans s^-1,excitation=488nm, emission=530-560nm). Fluorescence (F) was normalized to the resting fluorescence (F₀). [Ca²⁺]_i transients were analysed in terms of their amplitude (∆F/F₀), their full duration at half maximal (FDHM), and their full width at half maximal (FWHM). Data were summarized as the mean±SEM. P-values were determined using Student’s unpaired t-test. Under resting conditions, two main types of spontaneous [Ca²⁺]_i transients were observed in MVSM. Brief, spatially-localised events resembling Ca²⁺-sparks were often seen near the cell membrane, as well as more prolonged Ca²⁺-oscillations which spread across the full width of the cell. Sparks were observed arising from basal [Ca²⁺]_i levels with a frequency of 0.56±0.06 s^-1 (60 cells, 102 events). They had a mean amplitude (∆F/F₀) of 0.81±0.04, a mean FDHM of 23.6±1.2 ms, and a mean FWHM of 1.25±0.05 µm. Ca²⁺-oscillations occurred at an average frequency of 0.13±0.01 s^-1 (35 cells, 162 events). Their amplitudes were similar to those of the sparks (∆F/F₀ = 0.93±0.04), but they were very much longer in duration (FDHM = 1992±69 ms; P<0.001 v. sparks). Many oscillations appeared to result from the summation of Ca²⁺-sparks, and spark-like events were superimposed on the oscillations themselves. These appeared to originate from the same sites as basal-sparks in the same cells, but occurred with a much higher frequency (mean frequency=2.86±0.25 s^-1, 78 events; P<0.001 v. basal sparks). They had a smaller amplitude (∆F/F₀ = 0.69±0.04; P<0.05 v. basal sparks), a wider spread (FWHM = 1.67±0.07 µm; P<0.001 v. basal sparks), but were similar in duration to basal sparks (FDHM = 22.2±1.1 ms; N.S.). Thus, there were two distinct spontaneous spark populations arising from the same sites, presumably reflecting changes in localised Ca²⁺-release at different levels of cytoplasmic- and sarcoplasmic reticulum-[Ca² ]. The link between these changes and the mechanisms responsible for generation of the Ca²⁺-oscillations themselves remains to be determined.