The rate‐limiting step in the formation of prostanoids is the conversion of arachidonic acid to prostaglandin H₂ by cyclooxygenase, also known as prostaglandin G/H synthase/cyclooxygenase. Two forms of cyclooxygenase have been characterized: a ubiquitously expressed form (COX‐1) and a recently described second form (COX‐2) inducible by various factors including mitogens, hormones, serum and cytokines. Here we quantitate by the reverse transcriptase‐polymerase chain reaction (RT‐PCR) the expression of COX‐1 and COX‐2 mRNA in human tissues including lung, uterus, testis, brain, pancreas, kidney, liver, thymus, prostate, mammary gland, stomach and small intestine. All tissues examined contained both COX‐1 and COX‐2 mRNA and could be grouped according to the level of COX mRNA expression. The highest levels of COX mRNAs were detected in the prostate where approximately equal levels of COX‐1 and COX‐2 transcripts were present. In the lung high levels of COX‐2 were observed whereas COX‐1 mRNA levels were about 2‐fold lower. An intermediate level of expression of both COX‐1 and COX‐2 mRNA was observed in the mammary gland, stomach, small intestine, and uterus. The lowest levels of COX‐1 and COX‐2 mRNA were observed in the testis, pancreas, kidney, liver, thymus, and brain.