The aim of the present study was to determine the effect of different degrees of bulkiness in the gestation diet on reproductive performance of sows during their first two parities. The density of the diets was modified by incorporating a high proportion of fibrous ingredients (wheat bran and corn cobs in Diet 2 and oat hulls and oats in Diet 3). A diet without fibrous ingredients was used as a control (corn and soybean meal in Diet 1). Although all the gestation diets were formulated to provide the same daily intake of the major nutrients, sows fed Diet 3 were the lightest and had the lowest backfat thickness (P < or = .05) during both parities, and those fed Diet 2 were the heaviest and the fattest (P < or = .05) during the second parity. These results seem to indicate an overestimation of the calculated nutritive value of Diet 3, whereas those obtained with Diet 2 seemed to be linked to its marked effect on behavior of sows during the second parity. There was no marked effect (P > .11) of treatments on the number of pigs born alive, on preweaning mortality, or on the weaning-to-estrus interval. At parturition, total litter weight was similar among treatments in the first parity, whereas in the second parity it was greater (P < or = .05) in sows fed Diet 2 than in those fed Diets 1 and 3. In both parities, feed intake of the common lactation diet given during lactation was 5% higher (P < or = .05) for sows fed Diet 3 during gestation than for sows fed the other diets.(ABSTRACT TRUNCATED AT 250 WORDS)