A consistent set of equations has been written to describe the net rate of algal 14CO2 uptake (and where appropriate respiration and photosynthesis) which take into account separately complications due to respiration of the labelled photosynthetic products and the recycling of respiratory CO2. Written specifically into the equations is the concept of ‘new’ and ‘old’ carbon, the coefficient q is used in the respiration model to allow for the differential respiration of organic material from the ‘new’ and ‘old’ carbon pools. Analytical integrals have been found for respiration and recycling models, and the behaviour of the models studied over periods of 12 h (i.e. up to 70% of the intrinsic generation time). The rate constant for respiration has a greater effect on the behaviour of the recycling than the respiration model. Over short time courses (up to 30% of the intrinsic generation time), the effects of respiration and recycling on net 14CO2 uptake are quite distinct, especially at high P/R ratios, and not complicated by assumptions over the value of q. Although the value of q will have a time-dependent secondary effect on the modelled total carbon-specific respiration rate, this was found not to give rise to major problems of interpretation. Beyond 50% of the intrinsic generation time, the separate treatment of respiration and recycling in the models becomes less satisfactory. It was concluded that the present equations, which are not constrained by mass balance considerations, would not be appropriate for a model that combines the two processes. The pattern of recycling at low P/R values is identified as one of the major uncertainties in producing models of 14C uptake. The effect of the release of dissolved organic material can be anticipated in a general way. The models have been used to define an experimental strategy to establish the separate effects of respiration and recycling on the time course of net 14C uptake. The initial rates give the clearest resolution of the two processes and it would appear that with photosynthetic rates in the region of 1 day−1, incubation periods up to 3–6 h would be suitable to determine the importance of recycling in controlling net 14C uptake. With the present models, only in the absence of recycling could the effect of respiration be studied and the value of q established.