The regulation of the activity of purine transporters in two protozoan species, Crithidia fasciculata and Trypanosoma brucei brucei, was investigated in relation to purine availability and growth cycle. In C. fasciculata, two high-affinity purine nucleoside transporters were identified. The first, designated CfNT1, displayed a K(m) of 9.4 +/- 2.8 microM for adenosine and was inhibited by pyrimidine nucleosides as well as adenosine analogues; a second C. fasciculata nucleoside transporter (CfNT2) recognized inosine (K(m) = 0.38 +/- 0.06 microM) and guanosine but not adenosine. The activity of both transporters increased in cells at mid-logarithmic growth, as compared to cells in the stationary phase, and was also stimulated 5-15-fold following growth in purine-depleted medium. These increased rates were due to increased Vmax values (K(m) remained unchanged) and inhibited by cycloheximide (10 microM). In the procyclic forms of T. b. brucei, adenosine transport by the P1 transporter was upregulated by purine starvation but only after 48 h, whereas hypoxanthine transport was maximally increased after 24 h. The latter effect was due to the expression of an additional hypoxanthine transporter, H2, that is normally absent from procyclic forms of T. b. brucei and was characterised by its high affinity for hypoxanthine (K(m) approximately 0.2 microM) and its sensitivity to inhibition by guanosine. The activity of the H1 hypoxanthine transporter (K(m) approximately 10 microM) was unchanged. These results show that regulation of the capacity of the purine transporters is common in different protozoa, and that, in T. b. brucei, various purine transporters are under differential control.