1. The metabolic activities of the abdominal and scrotal testes have been studied in the guinea pig which had been rendered unilaterally cryptorchid by surgical procedures.
2. Histological observations as well as comparative differences in organ weights were used in the initial evaluation of the cryptorchid lesion, and the nucleic acid contents of the testes were estimated and correlated with the cellular composition of the organs.
3. Histochemical evidence indicated increases in the tubular and interstitial activities of the enzymes succinate, lactate and glucose-6-phosphate dehydrogenase in the cryptorchid guinea pig testis.
4. The histochemical localisation of glucose-6-phosphate dehydrogenase in the interstitial tissue of the testis, together with the increased activity of this enzyme during short-term retention of the organ, has been discussed on the basis of its functional significance in steroidogenic cells, especially in view of histological evidence of proliferative activity in the interstitium of the cryptorchid testis.
5. Quantitative estimation of enzyme activities showed significant increases in the levels of activity of glucose-6-phosphate and lactate dehydrogenases in the cryptorchid testis of the guinea pig after 3-4 weeks retention, but differences between scrotal and abdominal testes were not significant for succinate dehydrogenase during the whole of the experimental period.
6. Electrophoretio separation of the isoenzymes of lactate dehydrogenase was carried out in both the scrotal and abdominal testes of the guinea pig. The scrotal testis showed the presence of two additional isoenzymes (LD-X1 and LD-X2) for the guinea pig, which were not observed in the cryptorchid organ. These isoenzymes are shown to be identical to the "band X" isoenzymes of Blanco and Zinkham (l963), which are found only in spermatogenically active testes of some mammals.
7. The second "X-band" (LD-X2) of the guinea pig testes was lost from the abdominal organ after 1-2 weeks of retention, but the first "X band" (LD-X1) only disappeared after 3-4 weeks cryptorchism. These isoenzyme changes occurred in conjunction with severe tubular degeneration as shown in histological studies of the organ.
8. The cryptorchid testis also showed an increase in the proportions of A (muscle-type) subunits of lactate dehydrogenase which was closely correlated with the loss of LD-X (C subunit) activity. No changes were observed in the proportions of B (heart-type) subunits activity.
9. These changes in isoenzyme proportions, together with changes in total lactate dehydrogenase activity and the histochemical demonstration of its tubular predominance have been discussed in conjunction with other observations regarding the metabolic characteristics of the cryptorchid testis.