The rate, concentration-dependence and extent of histamine-evoked Weibel-Palade body (WPB) exocytosis were investigated with time-resolved fluorescence microscopy in cultured human umbilical vein endothelial cells expressing WPB-targeted chimeras of enhanced green fluorescent protein (EGFP). Exocytosis of single WPB was characterized by an abrupt increase in EGFP-fluorescence, morphological changes and release of WPB contents. The fluorescence increase was due to a rise of intra-WPB pH from resting levels, estimated as pH 5.45±0.26 (S.D., n=144), to pH 7.40. It coincided with uptake of extracellular Alexa-647, indicating the formation of a fusion pore, prior to loss of fluorescent contents. Delays between the increase in intracellular free calcium ion concentration evoked by histamine and the first fusion event were 9.9±3.9s (n=6 cells) at 0.3µM histamine and 1.6±0.2s (n=15 cells) at 100µM histamine, indicating the existence of a slow process or processes in histamine-evoked WPB exocytosis. The maximum rates of exocytosis were 1.20±0.16 WPB s^-1 (n=9) at 0.3µM and 3.66±0.45 WPB s^-1 at 100µM histamine (n=15). These occurred 2-5 seconds after histamine addition and declined to lower rates with continued stimulation. The initial delays and maximal rate of exocytosis were unaffected by removal of external Ca²⁺ indicating that the initial burst of secretion is driven by Ca²⁺-release from internal stores, however, sustained exocytosis required external Ca²⁺. Data were compared to exocytosis evoked by a maximal concentration of the strong secretagogue ionomycin (1µM), for which there was a delay between calcium elevation and secretion of 1.67±0.24s (n=6), and a peak fusion rate of ~10 WPB s^-1.