Testicular descent was prevented unilaterally in newborn rats by cutting the gubernaculum testis. Morphological changes due to non-descent of the testes were studied in 16-, 20-, and 24-day-old rats. Scrotal and abdominal temperatures were measured in control rats, and a difference was noted from 16 days and onwards. At 16 days of age the abdominal testes showed an abnormal accumulation of lipid droplets in the Sertoli cells at certain stages of spermatogenesis, and the number of tubules which had developed a lumen was slightly greater than in the scrotal testes. At 20 days of age, some spermatocytes were degenerating in the abdominal testes, especially in the tubular segments where lipid accumulation had been seen earlier. Lipid accumulation was noted in the Sertoli cells in all stages of spermatogenesis and additional ultrastructural signs of Sertoli cell malfunction such as dilatation of the SER and dilatation of the intercellular space between adjacent Sertoli cells was observed. Also the number of tubules containing a lumen was slightly larger in the abdominal testes. At 24 days of age, the number of spermatocytes was reduced in abdominal testes and the morphological changes seen earlier in the Sertoli cells were more pronounced. The function of the blood-testis barrier was investigated by the ability of the tubules to exclude lanthanum, and no differences were found between scrotal and abdominal testes at 16 and 20 days of age. The present study suggests that the earliest morphological changes in experimentally primary abdominal testes may occur in the Sertoli cells.