This study was undertaken to assess whether additions of different oils to the diets of male rats would affect the renal urokinase (UK) activity of healthy and pyelonephritic kidneys. Four groups of fatty acid diets were studied: fat-free, coconut oil, fish oil and evening primrose oil (EPO). Pyelonephritis was obtained by unilateral extrarenal urinary obstruction and subcutaneous injection of Escherichia coli. The UK activity of the non-obstructed kidneys did not differ statistically between rats infected and not infected with bacteria (P > 0.056), except within the coconut oil group. A statistically decreased UK activity was obtained with bacteria injected animals on a coconut oil diet (P < 0.0001). This phenomenon, namely a decrease in UK activity, was also seen with pyelonephritic kidneys of rats on fat-free, coconut and fish oil diets (P < 0.0065). However, the UK activity of the obstructed kidneys with and without infection in the EPO group remained similar (P = 0.8477). These results suggest that the UK activity in infection-induced renal stones may be restored by EPO containing diets and may be of high relevance in the prevention and treatment of infection-induced renal stones. This revelation now needs to be more fully investigated.