Analbuminaemic and Sprague-Dawley (control) rats were fed on low- (60 g/kg) protein and control (200 g protein/kg) diets ad lib. from weaning. Males and females were studied separately. Body-weight and plasma protein concentrations were determined at 10 d intervals from 25 to 75 d of age. Electrophoresis of plasma proteins was performed in samples from day 75. Extracellular fluid volume was measured at 10 d intervals from day 45 onwards. Colloid osmotic pressure was measured in plasma and interstitial fluid (wick technique) at the start and end of the trial. 2. Body-weight increased much less on the low-protein diet than on the normal diet in both strains and sexes. The growth retardation was slightly more pronounced in the male analbuminaemic rats than in the male Sprague-Dawley controls. 3. Plasma protein concentration increased during normal growth in all groups, particularly in the female analbuminaemic rats. This increase was reduced by the 60 g protein/kg diet in all groups, with the exception of the male analbuminaemic rats. 4. Differences in plasma colloid osmotic pressure were similar to those seen in plasma protein concentration. Interstitial colloid osmotic pressure was higher in the control rats than in the analbuminaemic ones. The interstitial colloid osmotic pressure increased during growth in the control but not in the analbuminaemic rats. The difference in interstitial colloid osmotic pressure between the strains was maintained during low-protein intake, but at a lower level than during normal protein intake. 5. Subtracting interstitial from plasma colloid osmotic pressure, resulted in a rather similar transcapillary oncotic gradient in the various groups at 75 d, both on the control protein diet (11-14 mmHg), and on the low-protein diet (9-11 mmHg). 6. All protein fractions were reduced to a similar extent by the low-protein diet in the control rats, whereas in the analbuminaemic rats protein fractions produced in the liver were more severely depressed. 7. Extracellular fluid volume as a percentage of body-weight was similar in all groups, and decreased with increasing age. 8. In conclusion, the analbuminaemic rats were able to maintain the transcapillary oncotic gradient on both diets by reducing the interstitial colloid osmotic pressure. Oedema was not observed. 9. Despite the absence of albumin, the protein-malnourished analbuminaemic rat is no more susceptible to hypoproteinaemia and oedema than its normal counterpart.
