3-O-Methyl-D-glucose transport in isolated adipocytes in the presence of insulin is symmetric in zero-trans experiments (transport into sugar-free solutions). K oi zt = 6.10 +/- 1. 65 mM, V oi zt = 1.20 +/- 0.19 mM/s; K io zt = 2.66 +/- 0.26 mM, V io zt = 1.19 +/- 0.07 mM/s. (The superscripts o and i and subscript zt refer to outside, inside and zero-trans conditions, respectively). In the absence of insulin K oi zt = 5.41 +/ 0.98 mM, V oi zt = 0.034 +/- 0.014 mM/s; K io zt = 4.09 +/- 1.05 mM, V io zt = 0.153 +/- 0.023 mM/s. For insulin pre-treated cells, infinite-cis experiments (transport into solutions of varying sugar concentrations) are also symmetric K oi ic = 6.51 +/- 0.83 mM, V oi ic = 0.09 mM/s; K io ic = 3.60 +/- 1.33 mM, V io ic = 1.76 +/- 0.63 mM/S (the subscript ic refers to the infinite-cis condition). In the absence of insulin, K oi ic = 9.03 +/- 3.28 mM, V oi ic = 0.066 +/- mM/s; K io ic = 4.54 +/- 1.32 mM, V io ic = 0.106 +/- 0.026 mM/s. The infinite-cis parameters are shown to be technically easier to measure than the zero-trans parameters. The uses of integrated rate equations for studying rapid transport are demonstrated. The results show that the adipocyte sugar transport system handles 3-O-methyl-D-glucose symmetrically, and that insulin does not change either the internal or the external affinity constants for this glucose analogue.