SUMMARYAt Onne in South‐east Nigeria, drainage water was collected from four monolith lysimeters and analysed for nitrate. The lysimeters contained an acid sandy loam. At the start of the first rainy season two lysimeters received urea labelled with15NO3–and two received no nitrogen fertilizer; all four were uncropped in the first year.The peak concentrations of15NO3–and of unlabelled (soil) NO3–were found after 2.5 pore volumes of water had passed through the lysimeters. Using the same soil in the laboratory after fine sieving, the peak concentration of tritiated water was found at 1 pore volume whereas nitrate leaching was retarded. The pattern of nitrate leaching was well described by miscible and immiscible models which included an adsorption coefficient for nitrate. Over the 2 years 81.4% of the15N added at the start of the first rainy season was recovered in the drainag