A mass balance approach was used to determine the factors influencing phosphorus and nitrogen dynamics in beaver ponds. The relationships of runoff, pond surface water temperature, dissolved oxygen (DO) and redox potential (ORP) to the annual and seasonal total phosphorus (TP) and total nitrogen (TP) retention of a headwater beaver pond situated on the Precambrian Shield, central Ontario, were examined during 1987-88. Annual retention of TP (-11%) and TN (-5%) were low. P and N were transformed within the pond. On an annual basis inputs exceeded outputs of total reactive P (71%) and NO3-N (35%) and outputs exceeded inputs of total unreactive P (-33%) and total organic N (-26%), while inputs approximated outputs of NH4-N (-8%). Marked seasonal trends in P and N retention were observed. Trends in monthly TP and TN retention showed a strong inverse relationship with runoff. There was a weak relationship between monthly retention and average water temperature and ORP. The timing of the major processes of nutrient cycling with seasonal variations in runoff and nutrient transport influenced the seasonal, and thus annual, TP and TN retention. Positive monthly retention coincided with low runoff and high biotic assimilation during the growing season. Winter ice cover was associated with undetectable DO and low ORP (<0 mV) and increased levels of P and N, particularly NH4-N (>800 ?g L-1). High levels of P and N in the water were coupled with increased runoff and potentially low biotic assimilation resulting in a net release of TP and TN during the winter. Large flow-through of waterborne inputs and flushing of regenerated P and N from the beaver pond occurred during peak snowmelt runoff, resulting in low annual retention. Estimates of burial rates suggest that P and N have accumulated in the pond sediments.