The objective was to equate the initial quality of the seeds (Ki), used in the probit model for predicting the longevity of corn (Zea mays) and soybean (Glycine max), as a function of the result of the initial tests of germination and accelerated ageing. Eleven seed lots of different corn and soybean cultivars were submitted to the tests of germination (TG), accelerated ageing (TEA), cold without soil (TF), for corn, and tetrazolium (TZ), for soybean, and to accelerated ageing tests, with constant conditions of moisture content and temperature of the seeds, to determine the value of Ki. Literature data with results of TEA and TF for corn and TEA and TZ for soybean were also used in this research. Simple and multiple linear regression analyses were conducted to equate: a) Ki as a function of TG and TEA for corn and soybean; b) TEAm (accelerated ageing of corn) as a function of TF; c) TEAs (accelerated ageing of soybean) as a function of TZ. The equations obtained were: Ki = 0,4230 + 0,4464 TG + 0,4131 TEA TEAM = - 0,4408 + 1,214 TF TEAS = - 0,1872 + 0,9875 TZ. The Ki equation presented an adjusted determination coefficient (r2) of 0,80 (d.p. 0,18 probit and parameters significant in p < 0,01, p < 0,02 e p < 0,05, respectively). The TEAm equation presented r2 of 0,80 (d.p. 0,24 probit and parameters significant in p < 0,002). The TEAs equation presented r2 of 0,84 (d.p. 0,18 probit and parameters significant in p < 0,005). The value of Ki can be equated as a function of the result of the initial tests of germination and vigor for corn and soybean (TEA or TF for corn and TEA or TZ for soybean).