A mass balance approach was used to determine the factors influencing phosphorus and nitrogen dynamics in wetlands common to headwater catchments of the Precambrian Shield. The relationships of runoff, water level, water temperature, and anoxia to the annual and seasonal total phosphorus (TP) and total nitrogen (TN) retentions of a headwater Sphagnum-conifer swamp during 1987--1988 were examined. Annual retentions of TP (4%) and TN (10%) were low in the swamp. On an annual basis, inputs exceeded outputs of total reactive P, NO3-N, and NH4-N and outputs exceeded inputs of total unreactive P and total organic N. Seasonal trends in P and N retention were inversely correlated with runoff. Positive monthly retention coincided with low runoff and increased biotic assimilation during the growing season. Water table drawdown during the summer was associated with peat aeration and increased levels of P and N in surface and pore water. High levels of P and N in the swamp surface water during the fall and winter were coupled with increased runoff, saturated overland flow, and potentially low biotic assimilation resulting in a net release of TP and TN. Large flow through of waterborne inputs and flushing of regenerated P and N occurred during peak snowmelt runoff resulting in low annual retention. ¿ American Geophysical Union 1993