We determined the effect of gestational age and labor on the regional expression of prostanoid receptor genes in baboon myometrium. Cesarean hysterectomy was performed on 15 pregnant baboons of known gestational age in the last third of pregnancy, five of them during spontaneous term labor. Expression of prostanoid receptor genes was studied using Northern blot analysis. Transcripts of similar size to the human were detected for prostanoid EP₁, EP₂, EP₃, EP₄, IP, FP, and TP receptor genes using Northern blot analysis. There were no gestational age-related changes in expression of these genes. Expression of EP₁, EP₃, and IP receptor RNA mRNA was significantly higher in myometrium from the fundus (compared with the lower segment), whereas EP₂ gene expression was significantly lower in the fundus. Labor was associated with a reduction in the regional variation of both EP₂ and IP receptor gene expression, but not EP₁ and EP₃ expression. Labor was also associated with an overall lower level of expression of EP₂ receptor mRNA. We conclude that regional and labor-related variation in myometrial expression of prostanoid receptor genes may have a key role in primate parturition.