The electrophoretic studies revealed that in non-pregnant and pregnant rats fed with protein-free diet (PFD), a significant increase in percentage of LDH1 and LDH2 (H-isozymes) was observed with a concomitant decrease in concentration of LDH4 and LDH5 (M-isozymes). The changes in LDH3 were not significant. Further, during protein deficiency heart-type activity (H-subunits) was increased markedly with a parallel decline in muscle-type activity; thus resulting in an increase in H/M ratio or analogue ratio (A.R.). However, the decrease in A.R. (below 2.00), observed during normal and steroid maintained pregnancy was due to higher muscle-type than heart-type activity. These observations suggest that protein malnutrition in rats favours a metabolic shift from anaerobic state to aerobic predominance in the uterus. The physiological significance of these adaptations has been discussed.