Steady state cultures of Anabaena flos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH(3), NO(3) (-), or N(2) gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (<0.03 per hour) in the NO(3) (-) and N(2) cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic O(2) evolution with growth rate was greatest when N requirements were met by NO(3) (-) and least when met by NH(3). These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH(3) cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO(3) (-)-and N(2)-grown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with NO(3) (-) and N(2) assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for NO(3) (-) assimilation were the active costs, while in N(2)-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with NO(3) (-) assimilation, N(2) fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N(2) assimilation.