This study was conducted in 1992 at the ESALQ/USP Seed Laboratory in Piracicaba (SP), Brazil, with the aim of characterizing possible relationships between a) seed size (width) and physiological quality, b) seed size and seed coat permeability and e) physiological quality and seed coat permeability. Samples of five commercial seed lots of Calopogonium mucunoides, an herbaceous tropical forage legume (aprox. 65 seeds per gram), were manually classified on overlayed round-perforated sieves into five size (width) classes (7,5/64?, 7/64?, 6,5/64?, 6/64? and 1/13?). Seed quality was evaluated by the standard germination and imbibition tests. For the imbibition test seed samples were placed in contact with moist blotters in closed plastic boxes, at 30°C; the percentage of imbibed (swollen) seeds were counted alter three, five and seven hours of imbibition. Seed origin (lots), sizes and its interactions had significant (p<0,05) effects on water absorption and physiological quality. Seeds of all sizes presented a similar pattern of water absorption but differed (p<0,05) in their rate of water absorption during the first three hours of imbibition; the smallest seeds (1/13?) were the fastest to imbibe possibly due to its high contents of immature and broken seeds. The hard seed content of the samples tended to increase as seed size diminished. Albeit a large variation from lot to lot, a general tendency was observed of the smallest seeds to present the lowest physiological quality, followed by the largest seeds (7,5/64?); intermediate size seeds were of best quality. The percentages of imbibed seeds resulted after three, five and seven hours and the rate of imbibition during the first three hours gave good estimation of the physiological quality of the seed samples, to which they correlate negatively. It was concluded that, in Calopogonium mucunoides, the remotion, via seed processing, of the small seed fraction may contribute to increase the quality of the seed lot but this decision should be based upon preliminary evaluation of its proportion and physiological quality.