Substance P immunoreactivity was measured by radioimmunoassay in aqueous humor samples of patients treated 13 h earlier with gentamicin, indomethacin or timolol. The indomethacin-treated group showed significantly decreased levels of substance P. The indomethacin effect is due to inhibition of the synthesis of arachidonic acid metabolites or involves non-specific mechanisms. It is suggested that suppression of substance P release may contribute to therapeutic effects of indomethacin in the human eye in conditions such as surgical miosis.