The mesometrial artery (MA), which is the final part of the uterine artery in guinea pig, supplies nutrients and oxygen to the fetus and placenta during pregnancy. Accompanying fetal growth, MA increases 8-fold in diameter, and 30- to 50-fold in weight, protein content and deoxyribonucleic acid (DNA) content. The importance of MA in the maintenance of pregnancy was examined by comparing the proliferative rate of MA during pregnancy and abortion induced by antiprogesterone RU38486 30 mg/kg. MA was incubated in Medium 199 with 185kBq/ml 3H-thymidine for 4h and the rate of 3H-incorporation into MA was used as the proliferative rate. The proliferative rate of MA during the first trimester (day less than 20) was 2,825 +/- 1,036 Bq/mg.h, which is almost 100 times higher than the rate in diestrus animals. During the later course of pregnancy, the proliferative rate decreased logarithmically, being 352 +/- 58 (40 less than or equal to d less than 50), 158 +/- 23 (50 less than or equal to d less than 60) and 75 +/- 10 (60 less than or equal to d). The proliferative rate of MA was measured also in animals which received RU38486 30 mg/kg at 24-48 h prior to the measurement and whose fetuses were still alive. The proliferative rates of MA in such animals decreased markedly to 141 +/- 21 (40 less than or equal to d less than 50) and 37 +/- 8 (50 less than or equal to d less than 60) (p less than 0.01), which means that the proliferation of MA is reduced even before the death of the fetus.(ABSTRACT TRUNCATED AT 250 WORDS)