Oxygen uptake through gills and skin has been measured in juvenile and adult Saccobranchus fossilis and its relationship represented by the equation Vo2=aWb. When air-breathing is allowed the O2 uptake via the gills and skin together increases by powers of 1 -084, 0–986 and 1–328 in juvenile, adult and juvenile+adult respectively. When air-breathing is prevented the slope (b) for O2 uptake via the gills appear to be less in juveniles (0–765) than in adults (0–784) and juveniles+adults together (0–814). Under the same experimental condition, the slope for O2 uptake via the gills+skin is also less in juveniles (0–478) than in adults (0–799) and juveniles+adults (0–755). Further decrease in the exponent value is found for the oxygen uptake of skin in relation to body weight under surfacing-prevented conditions (b= 0–542). Different exponent values for juvenile and adult fishes may be due to their different growth pattern and physiology.