Alterations in cytosolic calcium concentration influence a vast array of cellular functions in both excitable and non-excitable cells (Berridge et.al. 2003). [Ca²⁺]_i oscillations provide an additional level of control, permitting modulation of Ca²⁺-sensitive processes without the need for prolonged global elevations in [Ca²⁺]_i. Ca²⁺ oscillations have also been shown to increase the efficacy and specificity of [Ca²⁺] as a stimulus for the activation of certain transcription factors (Dolmetsch et.al. 1998). Bone marrow stromal cells are pluripotent precursor cells, which possess the ability to differentiate into a range of cell types including bone-synthesising osteoblasts. Studies have shown that 72% of commercially available human marrow-derived mesenchymal stem cells generate spontaneous [Ca²⁺]_i oscillations (Kawano et.al. 2002; Kawano et.al. 2003). We have studied the generation of [Ca²⁺]_i oscillations by primary rat marrow-derived stromal cells. Femora were removed from male Wistar rats humanely killed by cervical dislocation, and the marrow cavity flushed with culture medium. Marrow stromal cells were cultured in α-minimal essential media with 15 % [v/v] fetal calf serum (FCS), penicillin, streptomycin, ascorbic acid and β-glycerophosphate, 5 % CO₂ at 37 °C for 16 to 30 days. Cells were seeded onto glass coverslips and [Ca²⁺]_i was monitored by loading cells with Calcium-Green-1. Spontaneous oscillations were not observed in primary marrow stromal cells, (although occasional, irregular ‘Ca²⁺ spikes’ were seen in some cells). Exposure to 1 to 5% FCS, however, induced continuing regular Ca²⁺ oscillations in 38 ± 3.3% of cells (1703 cells; 49 experiments; ± SEM). These oscillations were blocked by disruption of Ca²⁺ stores with 100nM to 1μM thapsigargin. They were sensitive to inhibition of IP₃-receptor activity by 50μM 2-APB and to inhibition of phospholipase C with 5μM U73122. The oscillations exhibited a gradual decrease in amplitude and frequency following inhibition of Ca²⁺ influx with EGTA or La³⁺. Although the precise identity of the oscillation-inducing serum factor(s) are as yet unclear, preliminary experiments have revealed that the factor(s) are stable to heat (100°C; 4min), and are retained by a 30kDa molecular weight filter. Since serum is routinely present in cell culture medium at 10 to 15% [v/v], it is reasonable to assume that the observed oscillations occur constantly in these cells in culture. All procedures accord with current UK legislation.