1. The feeding rate of a whale is defined as the weight of daily food ingested expressed as percentage of body weight. Records of daily rations of Delphinoidea (porpoises and dolphins) in captivity allow calculation of feeding rates for eight genera having adult body weights of 10 to 10³ kg. 2. There is an inverse relation between feeding rate and body weight, both between species and between young and adult Delphinoidea. The range of feeding rates is from 12-13% to 4-6%. Above a body weight of 6x10² kg, represented by adult Tursiops truncatus, feeding rate remains constant at 4-6% up to the largest species in which it has been measured, the killer whale, Orcinus, of body weight 2 x 10³ kg. 3. Heart weight expressed as proportion of body weight varies directly with feeding rate and is equal to about one tenth of it. Young animals of all species, as well as adults of Phocoenoides dalli and possibly Phocoena phocoena have high feeding rates and heart weights, showing that both indices measure metabolic rate. From their heart weight/body weight ratio an attempt has been made to calculate feeding rates of whales too large to be kept in aquaria. 4. Heart weight/body weight ratios of adults rorquals of the genus Balaenoptera are about 4‰; of sperm whales, Physeter and one example of the small, related Kogia, lowest of all at 3.5‰,. These data suggest a feeding rate of 4% for adult rorquals, so that from curves relating body length and body weight, their daily food consumption may be calculated.