Fasting plasma levels of progesterone, glucose and insulin (IRI) were systematically investigated during a 20 days postpartum (p.p.) period in the same lactating (L) and non lactating (NL) rats. Progesterone levels, low at the end of gestation, remain low thereafter in NL groups; in L groups they increase on and after day 4 pp, reach values similar to the gestational ones, and decrease only after weaning. Glucose levels, low at the end of gestation, increase rapidly in NL group during the first 2 days pp and reach a stable value; in L group the increase is slower but the final stable value reached is the same. IRI levels, low at the end of gestation, increase similarly in both groups during the first 2 days pp then they return to low values in NL group whereas they remain elevated in L group until weaning (after which they decrease). It is suggested that the elevated IRI levels observed in L group despite normal glucose levels, are largely a consequence of the enhanced progesteronemia. 1 These studies were presented in part at the XXV International Congress of Physiological Sciences, July 15-31, 1971 in München (Germany) and at the 8th Annual Meeting of the European Association for the Study of Diabetes, September 6-8, 1972 in Madrid (Spain).