Observations at J odrell Bank of the intensity variations from ten pulsars are described. Two regimes are established. A pulse-to-pulse fluctuation is imposed at the source, simultaneously over a wide frequency range. Longer term variations show very fine frequency structure, the width of which varies between sources approximately as the inverse square of the dispersion measure. This slow fading is explained in terms of an interstellar scintillation model. Irregularities in the interstellar electron density with deviations of about 0·1 per cent of the mean density and scale of about 10 11 cm are deduced.