Summary. Age-related changes in daily sperm production (DSP) and testicular weights were investigated in paired testes from 89 men aged 21–50 years and 43 men aged 51–80 years. For both DSP/testis and DSP/g parenchyma, remarkably large standard deviations exceeded 50% of mean values. However, DSP/g and DSP/testis for both right and left testes were approximately 30% higher in the younger than in the older group (P < 0·01) and were negatively correlated with age (P < 0·01) when data from both groups were pooled. Weights of whole testes and of testicular parenchyma were similar in both age groups and were not significantly correlated with age. However, testicular tunic weights were 29% higher in the older group (P< 0·001) and were positively correlated with age (P < 0·001). Both testicular tunic weight and the % of total testis occupied by tunic were negatively correlated with DSP/g (P < 0·01); these correlations were weakened by removing the effect of age. Although total testicular weight and testicular parenchymal weight did not change with age, these values were about 10% lower on the left than on the right (P < 0·001). In addition to its increase with age, testicular tunic weight was about 8% greater for right than for left testes in all men (P < 0·001). Although the average size of the testis varied from right to left, DSP/g was similar in paired testes (P = 0·15), and the correlation between right and left DSP/g was high (p = +0·89, P < 0·001). Much of the variation in daily sperm production in men remains unexplained. However, sperm production does decline significantly with age in the human male.