The germination of developing seeds is very uncommon and is generally associated with deficiencies in abscisic acid (ABA) synthesis or sensitivity. This paper examines the quantitative relationship between the inhibition of precocious germination and endogenous ABA in the embryonic axis (ABAa) of hydrated soybean [Glycine max (L.) Merr.] seeds, isolated after the completion of histodifferentiation and before the beginning of dehydration, as well as the magnitude and evolution of axis sensitivity to endogenous ABA during that period. Developing seeds harvested at 25, 30, 35, 40 and 45 d after anthesis (DAA) were subjected to incubation or washing to induce changes in ABA content. ABA content was measured by radioimmunoassay, using a monoclonal antibody against free ABA. Germinability was measured as the time to 50% germination (t50). Washing and incubation induced eight- and twofold increases, respectively, in the rate of ABAa decline compared with the in planta ABAa decline. The threshold ABAa for inhibition of precocious germination (ABAc) increased slightly from 25 to 40 DAA [1.15–1.66 μg ABA (g DW)− 1]. This contrasted with the substantial decline in ABAa [10.90–2.07 μg ABA (g DW)− 1] during the same period, and indicated that sensitivity to endogenous ABA of hydrated seeds was initially high and diminished slowly during development. The relationship between (ABAa–ABAc) and t50 was linear for immature seeds incubated before and after washing. Below the ABAc, there were no differences in the t50 of 25–45 DAA seeds. The ABAa contribution to the control of precocious soybean seed germination was evident, although other potentially interacting factors were also present.