We have investigated the activity and kinetics of sodium-lithium counter-transport (SLC) in patients with IgA nephropathy and their relationship to plasma lipids. Standard SLC activity, the Michaelis constant (K_m) and maximum velocity (V_max) were measured in patients who had IgA nephropathy with either normal serum creatinine (IgA-NRF), or raised serum creatinine (IgA-IRF), and normal subjects (NC). The standard SLC activity was raised in hypertensive patients with IgA-NRF due to a raised V_max in association with hyperlipidaemia. The K_m was significantly lower and V_max also tended to be lower in IgA-IRF. K_m and V_max were not different in IgA-NRF compared with the NC. There was no difference in the mean standard SLC activity between all three groups. The low K_m and low V_max resulted in a normal standard SLC activity being observed in IgA-IRF which is similar to the situation we have observed in a proportion of diabetic patients with nephropathy. The low K_m in patients with IgA nephropathy may be due to inheritance associated with familial essential hypertension or to an acquired change of the kinetics related to a change in the environment of the plasma membrane during the development of renal impairment.