We have re-examined the effect of varying GDP concentrations on the kinetics of GTP-induced assembly of microtubules from microtubule protein, and on the elongation of pre-existing microtubules subjected to a temperature jump relaxation from 21.5 to 37 degrees C. The assembly kinetics follow a simple model for assembly which involves a fast equilibrium of tubulin-GTP and tubulin-GDP coupled to the elongation process due to tubulin-GTP. The initial rate of the relaxation process is found to be dependent upon the GTP/GDP ratio, in confirmation of the results of Engelborghs and Van Houtte (Biophys. Chem. 14 (1981) 195). As an alternative to the interpretation previously advanced by them, involving modification of the reactivity of microtubule ENDs by GDP, we show that this result is consistent with the above model with one reasonable modification, namely, that the ratio of the affinities of tubulin for GTP and GDP should vary with temperature. The analysis shows that a decrease in this ratio of approx. 2-fold at 37 degrees C accounts for the observed effects. We conclude that more complex mechanisms involving consideration of modification of the reactivity of microtubule ENDs by GDP are not required to explain these results. This finding has important implications for current models of GDP-induced microtubule disassembly.