Sertoli cells were studied using stereological methods in testes obtained from five children who were stillborn, and 31 individuals between 3 months and 40 years of age, who had suffered from sudden, unexpected death. The mean nuclear volume of the Sertoli cells, the numerical density of Sertoli cells, and the total number of Sertoli cells per individual were determined by point- and profile-counting of 0.5 micron sections. The nuclear volume of Sertoli cells increased from a median of 120 microns3 (range 53-130) during the period of 3 months to 10 years to 210 microns3 (170-260) in adults (greater than 25 years). The numerical density of Sertoli cells decreased from a median of 1200 X 10(6)/cm3 (870-1400) during childhood (3 months to 10 years) to 140 X 10(6)/cm3 (110-260) in adults (greater than 25 years). The total number of Sertoli cells per individual increased significantly from a median of 260 X 10(6) (130-520) during the late foetal period to 1500 X 10(6) (850-2900) in individuals from 3 months to 10 years of age. A further increase was found during puberty as the number of Sertoli cells in adults (greater than 25 years) was 3700 X 10(6) (2500-5600). These results indicate that significant qualitative and quantitative changes in the population of Sertoli cells take place after birth.