Photochemical smog episodes are frequent in Santiago of Chile city. We have measured the concentration of the main free radical precursors (formaldehyde, ozone, nitrogen dioxide, peroxyacetyl nitrate and nitrous acid), nitrogen monoxide and the irradiance. Calculations of the rate of OH* production were performed for different periods of the year and at emission (Cerrillos and Estacion Central) and receptor sites (Las Condes). From these data and simple steady state considerations were derived the concentrations of hydroxyl and hydroperoxyl radicals. Daily profiles of both radicals show a noticeable accumulation during early afternoon, the time of most intense oxidants accumulation. During summer time, hydroperoxyl radical concentrations are higher in Las Condes than in Cerrillos. The concentrations calculated for both radicals are higher than those estimated in other cities, suggesting a particularly high photochemical activity in the urban atmosphere of Santiago of Chile