1. The quantitative relationship between the suckling stimulus and several suckling-dependent variables was studied in lactating rats. 2. The intensity of the suckling stimulus, measured as the product of duration and number of pups suckling was in proportion to litter size for litters of one, ten or twenty-two pups. 3. Pups in the litters of ten grew fastest. Single-pup litters grew most slowly, some not at all. Litter weight gain, corrected for inevitable losses, was used to estimate milk yield. Total milk yield was greater for litters of twenty-two to twenty-four pups than for litters of ten pups only in the first week. Milk yield for single pups was up to 7% of the yield for ten pups. Milk yield per mammary gland sucked was similar for the larger litters but less for single pups. 4. The number of milk ejections, measured as pup stretch reactions, in observation periods of 225 min, was similar for litters of ten and twenty-two to twenty-four pups, but less for single-pup litters. 5. The suckling stimulus applied by single-pup litters inhibited ovarian growth less effectively than did the stimulus applied by larger litters. 6. There was no change in serum osmolality during lactation. 7. It is concluded that litters of ten or twenty-two to twenty-four pups applied suckling stimuli of different intensities, but these had similar effects on the daily secretion rate of oxytocin, milk production and suppression of ovarian growth. Single-pup litters were less effective.