1. We present a theoretical analysis of the tandem processes of transport and metabolic trapping which together constitute uptake of a substrate by intact cells. 2. Transport is assumed to occur by means of a simple carrier here analysed in its general form. Trapping is assumed to occur by a simple enzymic reaction. 3. We show how to obtain the separate parameters of the steps by analysing uptake data over a range of uptake times and substrate concentrations. 4. We present uptake data for uridine and cytosine-beta-D-arabinoside entering Nil 8 hamster fibroblasts, normal and murine sarcoma virus transformed, in the quiescent condition and after stimulation by added serum. We analyse the data in terms of the theory for tandem processes. 5. Transport is characterised by a system having a high Km and a high V for entry. The data for cytosine-beta-D-arabinoside suggest that the cytosine-beta-D-arabinoside system is not far from a symmetric one. The data for uridine transport do not differ when quiescent and serum-activated cells are compared. Transformed cells transport uridine at half the maximum velocity of normal cells, with or without added serum. 6. Trapping of cytosine-beta-D-arabinoside is insignificant. Trapping of uridine occurs by a system with both V and Km at least an order of magnitude smaller than are these parameters for transport. Trapping of uridine by non-transformed cells activated by serum, has twice the V of such cells in the quiescent state. 7. In the virus-transformed cells, the control of uridine trapping by added serum is lost, along with control of growth by this stimulant.