Respiratory characteristics of blood from sixteen adult male galagos were studied (8 brown and 8 black). The average body weight of the black galagos was significantly higher than that of the brown(1.512 +/- 0.162 vs. 0.988 +/- 0.099 kg, respectively). The blood oxygen capacities for black and brown galagos were 20.6 +/- 1.9 and 21.5 +/- 1.8 vol percent, respectively. The Bohr factor for blood from the black galago was -0550 +/- 0.039 (n = 9) and for brown galago blood was -0.523 +/- 0.050 (n = 6). There was no significant (P greater than 0.05) difference in P50 values of blood from the two subspecies of galagos, 36.7 +/- 1.1 vs. 37.7 +/- 0.9 mm Hg for the black and brown, respectively. The Haldane effect for black galago blood was higher than that for brown (7.5 vs. 65 vol percent, respectively). The concentrations of 2.3-DPG were similar in black and brown galago blood, 16.16 +/- 1.52 vs. 16.37 +/- 0.76 mumol/g Hb, respectively. Starch gel electrophoresis showed three major components of hemoglobin from both subspecies. There is a trend toward decreasing blood oxygen affinity as one ascends the primate evolutionary scale, which may be secondary to the trend toward a greater adult body weight.