To examine genetic variation in the growth curve parameters and their contribution to factors determining grain yield, 40 randomly chosen F 7 lines of a cross between two contrasting rice varieties were measured for dry matter weight and for other yielding characters during the growing period. From the dry weight data, growth curves were computed using linear and quadratic (Pearl-Reed) logistic equations, and various values derived from them were compared among lines. The results showed that: 1) The growth curves of the lines varied genetically between the "early-vigor" and "sustained-vigor" types. 2) Total panicle length per unit area, representing the potentiality for carbohydrates to be produced in grains, seemed to depend on the growth rate at floral initiation, and panicle weight per unit length, representing the rate of carbohydrate supply to the panicles, seemed to depend on the growth rate at heading. These two values were negatively correlated. 3) "Early-vigor" types tended to produce more panicles but had a low rate of carbohydrate supply than "sustained-vigor" types. Either "potentiality" or "supply" was a limiting factor in grain production, depending upon the pattern of growth being either "early-vigor" or "sustained-vigor" type.