AbstractThe effects of maternal hyperthermia on the prenatal development of the guinea pig lens is described with particular reference to cataract formation. Cataracts were observed at birth most commonly following maternal hyperthermia in early pregnancy (E11‐25) but also at mid‐pregnancy (E39‐46) and in late pregnancy (E56‐60). Cell death was observed in the lens and neural retina up to 8 hours following heating at E21 and vacuoles were frequently found in the lens fibres immediately beneath the anterior lens epithelium. After heating at E18 and observation at E26 the vesicles in the anterior part of the lens fibres were of varying sizes with the smaller appearing to coalesce into larger vesicles. These vacuoles may be an example of compromised lens fibres whose cellular changes may remain latent and form cataracts in later life following any event which alters fluid balance of t