The isoenzymes carbonic anhydrase II (CAII) and III (CAIII) have been measured by radioimmunoassay in the livers of male and female rats aged from 21 to 800 days. No sexual dimorphism at 21 days was found, but from 50 to 400 days both isoenzymes show sexual differences. From 600 days onwards, these differences are less apparent. CAIII concentrations in two 'fast' fibre muscles and one 'slow' fibre muscle have been determined. There is no sexual dimorphism in muscle, but a wide variation between individuals was observed. Fast muscles show maximal CAIII levels at 800 days, whereas in slow muscle the concentration of the isoenzyme is declining at this time.