The oil content of soybean (Glycine max (L.) Merrill) seeds is a polygenic and complex trait that is responsive to environmental effects that occur during plant development. Our objective was to study the seed oil content of soybeans developed under diverse photoperiod and temperature conditions. Three parental inbred lines with classic (BR-13, FT-2 and BR85-29009) and one with long juvenile flowering type (OCEPAR 8) and the F2, F3, and F9 generations derived from all possible crosses between them (including reciprocals) were sowed in September 27th, October 20th and December 17th in 1993 in Londrina, Paraná State, Brazil (between 23o08'47" and 23o55'46" latitude S). The October and December sowing dates are within the period the varietal research personnel recommend for sowing soybeans in Paraná State. The analysis of variance indicated significant differences among sowing dates, among advanced inbred lines, and the sowing date x inbred line interaction. Seed oil content increased from September to October and decreased from October to December in all materials, but the reduction was greater in FT-2 and OCEPAR 8 among the parentals. The additive genetic variance (D) or additive variance among linked genes (D1) was significant for all crosses and sowing dates. Genotype x micro-environment interactions were important in some crosses. The additive [d] effects were greater in September and October, and the additive x additive interaction [i] was important in October among the mean genetic parameters. Significant dominance effects [h] were more frequent in December and October, often in direction of the increased seed oil content. The heritability estimates ranged from 15 to 43%, with the highest values obtained in September. The prediction of cross potential to generate higher seed oil inbred lines indicated that selection is likely to be successful in most crosses. The highest proportion of inbred lines with seed oil percentage above the standard (lines with more than 22% seed oil content) was for BR85-29009 x OCEPAR 8 in September, FT-2 x OCEPAR 8 in October, and in BR85-29009 x OCEPAR 8 and BR-13 x OCEPAR 8 in December.
