The concentration of Na,K-ATPase in biopsies of uterine muscle was determined by measurement of [³H]ouabain binding in the presence or vanadate. For this purpose a method previously described for skeletal muscle (Norgaard et al. 1983) was modified. Biopsies were obtained from uterine muscle from pregnant women (during caesarian section), non-pregnant women (during hysterectomy) and from adult, non-pregnant guinea-pigs and rats. The ouabain binding site concentration in uterine muscle of the pregnant women averaged 72 ± 2 pmol g^-1 wet wt (n = 8), with an apparent dissociation constant (K(D)) for ouabain of 3 x 10^-9 mol l^-1. The ouabain-binding capacity in uterine muscle of the non-pregnant women amounted to 83 ± 9 pmol g^-1 wet wt (n = 8). In uterine muscle of the guinea-pig, two populations of ouabain binding sites were observed: one with a maximum binding capacity of 230 pmol g^-1 wet wt and an apparent K(D) of 1.6 x 10^-6 mol l^-1, and one with a maximum capacity of 62 pmol g^-1 wet wt and an apparent K(D) of 5 x 10^-8 mol l^-1. Immediate freezing of the biopsies in liquid N₂ and storage at -60°C for up to 6 weeks caused no change in ouabain-binding capacity. The dry weight/wet weight ratio of the samples from different subjects showed values of around 20%. It is concluded that the concentration of Na-K pumps in human uterine muscle can be quantified by [³H]ouabain binding using samples weighing 5-10 mg. The method may provide a useful approach in clinical studies, i.e. analysis of the changes in Na, K-ATPase concentration that may occur as a basis for disturbances of labour.