Livestock waste represents a potentially important source of methane (CH4) emissions, but little experimental data are available on CH4 production from manure stored on farms. The objective of this study was to determine the effect of slurry characteristics (high and low total solids (TS) contents), temperature (10 and 15 degrees C), and storage duration (180 and 272 days) on methane emissions from dairy cow and swine manure slurries. The manure was stored in closed 232-L plastic barrels simulating the anaerobic conditions of a farm-scale storage tank. Storage conditions and manure characteristics were typical of Canadian commercial livestock farms. Over the 272-day storage period, barrels filled with swine manure produced from 1.32 to 3.81 L CH4 per L of manure (L/L) at 10 degrees C and from 7.34 to 7.43 L/L at 15 degrees C. When evaluated on a per kg of volatile solids (VS) basis, methane production at 15 degrees C was significantly higher at low (205.1+or-25.58 L/kg) than at high (89.49+or-25.62 L/kg) TS contents. Total methane production from dairy cow manure stored at 10 degrees C ranged from 0.26 to 0.28 L/L. At 15 degrees C, average methane emissions from dairy cow manure significantly increased from 0.33 L/L at high TS to 3.77 L/L at low TS content. Methane emissions from low TS dairy cow manure stored at 15 degrees C were mostly recorded between days 180 and 272, due to an increase in both biogas production and methane content of the biogas. Under similar storage conditions, swine manure produced more methane than dairy cow manure. Results suggested that CH4 emissions depend on the interaction between a number of variables, including physico-chemical characteristics and type of manure, temperature, and storage duration. Results also indicated that on typical Canadian farms, CH4 emissions from manure storage tanks over the late fall, winter, and early spring period should be very small, because manure temperature remains substantially below 10 degrees C. However, manure slurry should be removed daily or weekly from swine buildings, where manure temperature is maintained above 10 degrees C. During the late spring, summer, and early fall period, CH4 emissions from manure tanks could be substantially reduced by recommending storage periods shorter than 150 days and frequent land applications. The use of below ground storage tanks would also contribute to maintain lower manure temperatures during the summer and thus minimise CH4 emissions.