We have demonstrated the role in some CAPD patients with ultrafiltration (UF) loss of an increased peritoneal lymphocyte (PLy) and macrophage (PMO) Ca++ concentration in the release of large amounts of gamma-Interferon (gamma-IFN) and Interleukin-1 (IL-1), which stimulate peritoneal fibroblast proliferation. We have also shown in vitro and in vivo that the calcium channel blocker verapamil (VPM) is able to normalize the previously high Ca++ PLy and PMO concentration and cytokine release, to decrease fibroblast proliferation, and to increase UF in only 60% of the CAPD patients with UF loss due to a cytokine-mediated hyperproliferation of peritoneal fibroblasts, while in the remaining 40% there is little improvement in UF (VPM responders and low-responders, respectively). To evaluate which mechanisms in addition to passive Ca++ influx can play a role in the Ca(++)-dependent activation of peritoneal immune-cells, we evaluated in 6 CAPD VPM low-responder patients the effects of in vitro of different doses of Ca++ and 1,25(OH)2D3 on: 1) PLy and PMO cytoplasmic Ca++ levels in the PLy and PMO cytoplasm; 2) gamma-IFN and IL-1 release by PLy and PMO; 3) peritoneal fibroblast proliferation. Results showed a direct correlation between Ca++ levels in the medium and the PLy and PMO Ca++ concentrations, IL-1 and gamma-IFN release, and peritoneal fibroblast proliferation. These effects were enhanced by the addition of low doses of 1,25(OH)2D3 to the medium, while both high 1,25(OH)2D3 doses and verapamil abrogated the Ca+(+)-induced PLy and PMO activation. These results underline the importance of both Ca++ and 1,25(OH)2D3 in peritoneal immune-cell activation and peritoneal fibroblast proliferation, and may offer a new prophylactic approach for preventing UF loss in CAPD.