Recently, fat-hen (Chenopodium album L.) biotypes resistant to metamitron, a key herbicide in sugar beet, were recorded. Pot experiments revealed that these biotypes showed cross-resistance to metribuzin, a triazinone used in potato. Greenhouse and laboratory experiments were performed to develop resistance monitoring tests, so that resistant biotypes can be detected quickly and farmers may adapt their weed management. Resistant and susceptible biotypes were grown in a greenhouse under conditions of natural and artificial light at an intensity of 100 micromol photons m(-2) s(-1). Leaves were collected and, immersed in a solution of 1000 microM metamitron and 500 microM metribuzin, exposed to natural and artificial light (1000, 750 and 100 micromol photons m(-2) s(-1) respectively). After this, chlorophyll fluorescence measurements were carried out. The results revealed that the photosynthetic electron transport of metamitron- and metribuzin-incubated leaves of resistant biotypes decreased less than that of the incubated Leaves of susceptible biotypes. The differences between the metribuzin-incubated leaves of the susceptible and resistant biotypes were larger than those observed with the metamitron-incubated leaves. The aim of the experiments was to optimise the chlorophyll fluorescence test and to find a sufficiently high correlation between the results of the pot experiments and the chlorophyll fluorescence measurements.