This reasearch, carried out within the MON.I.TO. programme (Intensive Monitoring of Forests in Tuscany, central Italy), was aimed at defining the stress conditions of two Quercus ilex woods at Colognole (CL) and Cala Violina (CV). In vivo chlorophyll fluorescence was measured for a whole year (1997 - 1998) using a portable 'continuous fluorescence' fluorimeter. The fluorescence parameters were calculated using the values at the steps 0-J-I-P of the polyphasic rising transient (this analysis is called JIP-test). Measurements were performed monthly, pre-dawn and midday. The findings revealed a condition of stress (drought and high temperatures) in the summer at CV during the central hours of the day, and a winter cold stress condition at CL observed in the pre-dawn measurements. At CV, summer stress was combined with processes of biologically regulated de-excitation; at CL, the winter stress appeared to be linked to structural damage affecting the photosynthetic apparatus. Results are discussed in relation to the existing literature on stress affecting Mediterranean vegetation, and to the findings reported in previous investigations in the same study areas.