Metabolism and insulation of monk parakeets were determined in summer and compared with previously reported winter values. Fasting levels of metabolism were the same in winter and summer during the daytime: nighttime values were significantly lower in winter (6.11 versus 8.20 Wkg-1). Evaporative water loss at 20-30° C was significantly lower in winter (2.24 versus 3.05 mg H2O g-1 h-1). Winter birds were better able to maintain body temperature (Tb) independent of ambient temperature (Ta) below 20° C than were summer birds. Coefficients of dry heat transfer determined for Tb-Ta gradients greater than 10° C were not significantly different for the two seasons. Similarly, the slope of the line relating oxygen consumption to Ta below thermal neutrality (i.e. thermal conductance) was the same in winter and summer birds, indicating no seasonal change in overall body insulation. Literature review revealed a relation between seasonal change in fasting metabolic rate (FMR) and body size in birds. Winter FMR as a fraction of summer FMR is inversely, related to body mass (Mb) by the equation; winter FMR/summer FMR=1.49 Mb -0.077 where Mb is in grams. Thus, in small birds FMR is higher in winter than summer, while for large birds (>200 g) the converse is true.