The extinction profiles as well as the hyper Rayleigh scattering signal of a highly monodisperse 22 nm diameter gold colloidal sol in the presence of pyridine have been measured. A strong enhancement of the HRS signal is observed in the presence of small amounts of pyridine. The relationship between the extinction spectra, the distribution of aggregated particles, and the second harmonic response has been investigated by evaluating the enhancement factor of the electromagnetic field in the colloids as they slowly aggregate. It is also shown that at larger pyridine concentrations, the hyper Rayleigh scattering signal levels off owing to the formation of large aggregates.