Ground-water quality samples were obtained at regular intervals from 10 domestic water wells over a 21-month period, and were analysed for the common inorganic chemical constituents present in the natural hydrogeological environment. The purpose of the sampling study was firstly to determine the degree of fluctuation over time in each ground-water quality parameter, and secondly to provide sampling frequency input data for the planning of a permanent ground-water quality monitoring network in Ontario. Parameter fluctuation was assessed with respect to the different types of aquifers or water-bearing formations found in southern Ontario, and with respect to the different depths and types of water wells common to the province. Wells varied in depth from 13 feet to 228 feet below ground level, with eight wells obtaining water from overburden aquifers and two from bedrock aquifers. Four wells located in the same area but obtaining water from different aquifers were also included to determine if correlations in water quality were apparent in different formations at the same location. Data analyses using the coefficient of variation were valuable in assessing the relative degree of variability of each study parameter and of each well. The parameters total iron and total nitrate were found to be the most variable, whereas hardness, alkalinity and pH were the least variable. Seasonal variability was found in some cases for nitrate and chloride. Random variability was found in total iron and to a certain extent in specific conductance. Wells obtaining water from confined aquifers showed a lesser degree of variability in water quality than wells obtaining water from leaky or unconfined aquifers. Sampling frequency in a water-well network setting was established at one or two samples per year for most parameters with the exception of iron and nitrate, which would require more frequent sampling in order to adequately assess the concentrations.