The extent of enhanced post-illumination respiration (EPIR) has been investigated in a number of microalgae. Respiration rates, as determined by O2 consumption, were enhanced (in all but one case) by 50-140% following pre-exposure to high photon flux compared to rates obtained for steady-state dark respiration. The extent of EPIR was dependent more on photon flux than on duration of exposure, although the latter did have some effect. In Isochrysis galbana and Chaetoceros calcitrens, EPIR effects were also demonstrated using [14C]CO2 evolution. In I.galbana, release of CO2 from cells pre-exposed to a period of high photon flux was most rapid from carbohydrate and low-molecular-weight metabolites. Data obtained from Thalassiosira weisflogii indicate that cells grown at low photon flux are more susceptible to EPIR than those grown under high photon flux. These results are discussed in the context of various hypotheses that have been proposed regarding the mechanism of EPIR effects.