We calculated energy budgets for lactating fat sand rats (Psammomys obesus) and their pups. Females increased metabolizable energy intake (MEI) over maintenance requirements (ME(m)) by up to 85% in small (one to three pups), 118% in medium (four to five pups) and up to 138% in large (six to eight pups) litters. In the first half of lactation (days 1-15) there was an increase in body energy of females by 8 and 62.5 kJ in medium and large litters, respectively, and a decrease of 74.5 kJ in females of small litters. In the second half of lactation (days 16-30) body energy of females decreased by 136, 206 and 191 kJ in small, medium and large litters, respectively, which contributed to 30-45% of milk energy production. Milk intake per pup was significantly higher in small than in large litters. In all litters, milk intake per pup increased with time and then decreased towards weaning. Maximum energy retention of pups during growth occurred in small litters. From 15 days of age, pups of all litters started to feed on Atriplex halimus leaves. Pups increased their MEI from A. halimus rapidly and concomitantly reduced their milk energy intake. Just before weaning, MEI from A. halimus reached 90%, 91% and 93% for small, medium and large litters, respectively. Efficiency of lactation is usually expressed as either (i) body energy increase in young as a fraction of the additional MEI above ME(m) of females or (ii) body energy produced in the litter as a fraction of milk production. Both methods do not determine the efficiency of milk energy intake for growth for they do not separate ME(m) of pups from heat increment of milk energy for growth. We used a two component non-linear model to calculate ME(m) of pups and their efficiency of utilization of milk for growth of the pups. Efficiency of utilization of milk for growth was 85-91% for the pups and therefore 9-15% of the metabolizable energy of milk, above ME(m) of the pups, was lost as heat increment of feeding. No such values are available for other rodents.