An automated flying spot microscope has been used to estimate the volume proportion of collagen gel in myocardium. Observations were limited to the interventricular septum and 40 specimens were examined, nearly all from normal hearts in the age range 13—92 yr. A small but significant increase with age was found for collagen gel in regions in which the muscle fibres were predominantly transverse — that is, in the subendocardial and subpericardial zones — although there was no significant increase where they were predominantly longitudinal — that is, in the central myocardial zone.