Testicular parameters, parenchymal spermatid concentration, and epididymal sperm reserves were measured in tracts obtained from 16 Awassi rams classified into three age groups (1, 1.5 and 2.5 years old). Between 1 and 2.5 years of age the paired testis weight increased from 164.9 to 315.2 g, the testicular spermatid number (X 10(9)) increased from 15.9 to 55.8, and the total epididymal sperm reserves (X 10(9)) increased from 11.1 to 98.8. Significant correlations were found between the paired testis parenchyma weight and all testicular dimensions and size. Testicular spermatid numbers and concentration were correlated with paired testis weight (r = 0.91 and 0.63), paired testis volume (r = 0.92 and 0.64) and all testicular measurements, particularly paired length X depth (r = 0.92 and 0.66). Regression coefficients between all parameters were essentially linear and highly significant, allowing accurate predictions of testis weight, volume and spermatid numbers to be made. The total epididymal sperm reserves and the sperm content of each of the caput, corpus and cauda of the epididymis were correlated with the paired weights of the epididymis (r = 0.73, 0.75, 0.73 and 0.70), the testis (r = 0.76, 0.83, 0.91 and 0.72), the total spermatid numbers in testicular homogenates (r = 0.70, 0.85, 0.91 and 0.64), and spermatid concentration (r = 0.56, 0.75, 0.72 and 0.49). The storage capacity of the cauda epididymides appeared to develop more slowly than the rate of sperm production because at 2.5 years of age it was about 1.5 and 2.8 times that in rams aged 1.5 and 1 year respectively.