Studies of dermal-epidermal interactions were conducted with guinea pig flank skin and intradermal injections of the irritant, Sudan IV dye in olive oil. These injections led to epidermal hyperplasia in areas overlying the irritant and the effect was most significant when the irritant was placed in the upper dermis. Basal cell mitotic activity and thymidine uptake reached a peak by 24 h and thereafter dropped rapidly. Maximal epidermal thickness (4.3 times the control) resulting from an increase in cell number occurred within 2–4 days. Despite the very short period of increased cell growth, epidermal thickness returned to control values only after a 24-day period. A similar growth response could not be induced by saline injections. A single topical application of the irritant showed a qualitatively and quantitatively different epidermal response. These experiments indicate that an intradermal irritant can lead to epidermal hyperplasia and a long-lasting epidermal thickenin