Selection for yield has resulted in an annual 0.5% genetic gain in soybean ('Glycine max' [L.] Merr.) since the 1930s. Comparisons of older, low-yielding cultivars and newer, high-yielding cultivars should elucidate those traits associated with genetic gain in yield. The objective of this study was to determine the physiological traits associated with yield improvement of short-season Ontario soybean cultivars. Field trials were conducted in 1996 and 1997 at the Elora Research Station as a split-split plot experiment with four replications each year. The main plot was cultivar, the sub-plot was a source-sink treatment and the last split was harvest date. Two pairs of old and new cultivars with similar maturities were grown under identical agronomic practices. The source-sink treatment consisted of a control and a 50% depodding treatment at R4, which resulted in a 20-25% reduction in pod number by maturity. Harvests were taken periodically from the vegetative stage to maturity (V5, R1, R4, R5, R6, R6.5, R7 and R8). Dry matter and nitrogen (N) accumulation as well as leaf area indices (LAI) were measured. SPAD chlorophyll values were recorded at around R6. In an analysis of results using only the full pod load treatments, the newer cultivars had a higher crop growth rate (CGR) and slower rate of decline in LAI during the seed filling period (SFP) than older cultivars. All cultivars accumulated similar amounts of N until the beginning of the SFP. After the onset of the SFP, the newer cultivars accumulated more N than the old cultivars. Source-sink ratio as measured by CGR/SGR (seed growth rate) or as the amount of assimilates re-mobilized out of the vegetative tissue (in order to maintain seed growth), suggested that newer cultivars had higher source-sink ratios during the SFP. Analysis of the response to the pod load treatment, it was observed that N accumulation was correlated to assimilate availability during the SFP. The depodding treatment increased source-sink ratio during the SFP and reduced the rate of leaf senescence in the older cultivars more than in newer cultivars. Higher yields of newer vs. older soybean cultivars were associated with increased source-sink ratios and longer leaf area duration during the SFP.