A single crystal of an Al-Mg-Si alloy (Mg: 0.43 wt%, Si: 0.47 wt%) was aged at 453K while small-angle neutron scattering experiments were carried out in situ. The scattering data recorded on a two-dimensional detector show the symmetry typical for needle-shaped precipitates oriented in the three [100] directions of the aluminium lattice and allow the calculation of the length, diameter and number density of the precipitates assuming cylindrical scattering objects of equal size and composition. The repetition time of the experiments was approximate to 12min. The values obtained for the three quantities agree well with values measured by transmission electron microscopy (TEM) on a similar alloy. The impact of one week of natural ageing before artificial ageing on the evolution of the size and number density of precipitates is found to be pronounced, as expected from published TEM data.