We have recently developed a mass spectrometric disequilibrium technique to measure CO2 and HCO3- uptake during steady-state photosynthesis in cyanobacteria and microalgae (Badger et al. 1993). Using this technique, we have found differences between the characteristics of Ci uptake in Synechococcus PCC7942 under steady- and non-steady state photosynthesis. Both low- and high-Ci cells have the capacity to transport CO2 and HCO3-. In contrast to reported results obtained under non-steady-state conditions, the activity of HCO3- uptake is higher than that of CO2 uptake, even in high-Ci cells. Dramatic increases in the affinity of the CI transport system for CO2 and HCO3- but not in maximal transport activities were found when high-Ci cells were induced under low-Ci conditions. These changes in affinity occurred within about 4 h. The affinity for HCO3- but not CO2 also increased when uptake activities were measured at a lower pH (pH 7.5 compared to pH 8.5), especially in high-Ci cells. HCO3- uptake was also more sensitive to LiCl inhibition than CO2 uptake. LiCl inhibition was partially relieved in the presence of NaCI. The carbonic anhydrase inhibitor ethoxyzolamide inhibited both CO2 and HCO3- uptake to about the same extent, consistent with the view that both transports might share a common 'carbonic anhydrase-like' component. The results are discussed in relation to previous results obtained under non-steady-state conditions.