We conducted several experiments on linuron-resistant and -susceptible Portulaca oleracea and on atrazine-resistant and -susceptible Chenopodium album to determine their immediate and long-term responses to photosynthesis-inhibiting herbicides. Several photosynthesis-inhibiting herbicides were used, and O2 evolution was measured with a Clark-type O2 electrode. Resistance ratios (RRs) for P. oleracea, based on O2 evolution inhibition, were 8 and > 6 for linuron and diuron, respectively; > 800 for atrazine; and > 20 for terbacil. Linuron-resistant P. oleracea was negatively cross-resistant to bentazon and pyridate (RR = 0.5 and 0.75, respectively). Time-course measurements of fresh weight, photosynthetic CO2 assimilation, and photochemical efficiency indicated that linuron and atrazine inhibited electron transport in susceptible (S) P. oleracea and C. album, ultimately resulting in death. Measurements of photochemical efficiency and CO2 assimilation of linuron-resistant P. oleracea treated with linuron indicated a transient injury from which plants recovered within 14 d. Recovery of linuron-resistant P. oleracea from atrazine injury was more rapid than from linuron injury for all measured variables. Atrazine-resistant C. album had no cross-resistance to linuron. Sequence analysis of the D1 protein revealed that linuron-resistant P. oleracea had a serine-to-threonine substitution at position 264.