The results presented here update those of Casey and Clarke (Freshwater Biology, 1979), ecologists who studied nitrate concentrations in the River Frome in southern England from 1965 to 1976, finding that average annual nitrate concentrations were increasing at the rate of around 0·11 mg NO3N l−1 year−1. Statistical analyses at the same East Stoke site for the period 1978 to 2003 are presented and results are compared with the neighbouring River Piddle for the period 1978 to 2007 which, unlike the Frome, lies entirely within the Chalk in its upper reaches. The method of Casey and Clarke (1979) is followed in applying general linear and nonlinear estimation models (NEMs) to explore the sources of variation in the nitrate concentration data. The proportions of the total variation in nitrate concentration attributable to differences between years and to differences between months are similar for the Frome in both periods. For the Frome and Piddle models from 1978 onwards, an even greater proportion of variation is explained by differences between years rather than differences between months, suggesting that long-term change is the dominant effect. Analyses of annual mean nitrate concentration data show a sustained linear trend over the study period for both rivers and indicate that the rate of increase of annual mean nitrate concentration continues at a rate not significantly different from that found by Casey and Clarke (1979). Four versions of a NEM were applied, including the original Casey and Clarke (1979) model. The inclusion of flow variables adds little to the overall explanation and a cosine index remains the most effective descriptor of seasonal variations in nitrate concentration, suggesting seasonal in-stream biological uptake as a dominant control, rather than hydrological catchment response. Copyright © 2008 John Wiley & Sons, Ltd.