The analysis of chlorophyll precursors in tobacco (Nicotiana tabaccum L.) and barley (Hordeum vulgare L.) plants that were grown in light/dark cycles revealed diurnal fluctuations of the levels of magnesium protoporphyrin (MgPP) and its monomethyl ester (MgPPMe). Based on the dark levels, a steep increase of both MgPP and MgPPMe was observed at the beginning of the light phase, culminating at 30 min (tobacco) and 60 min (barley) of illumination. This was followed by a sudden decrease within about 1 h and a broad second maximum at about 8 h after start of illumination. The increase at the beginning of the light phase, based on the dark level, was 100-fold (MgPPMe, tobacco), about 200-fold (MgPPMe, barley), 60-fold (MgPP, tobacco) and 30-fold (MgPP, barley), respectively. The metal-free protoporphyrin was not detectable in barley plants. In tobacco, its level showed a slow increase until 8 h of illumination followed by a slow decrease until the end of the light period. Light-dependent changes in the metabolic flux of chlorophyll precursors are discussed as possible mechanisms of the observed fluctuation.