1. An increase in glomerular prostaglandin (PG) synthesis has been reported in two experimental models of increased glomerular filtration rate (GFR).

2. Because pregnancy, in women and rats, is also associated with an increased GFR, we studied PG biosynthesis by glomeruli isolated from pregnant rats in comparison with virgin rats at the different phases of the oestrous cycle.

3. PGE2 and PGF synthesis markedly increased during pregnancy (at day 21 of gestation a five- to sevenfold increase of PGE2 and PGF was seen; at day 15 of gestation a threefold increase of PGE2 and a sixfold increase of PGF was noted). However, thromboxane A2 (TXA2) synthesis was similar in all the groups studied.

4. Among the four phases of the oestrous cycle, dioestrus was characterized by a significant increase in glomerular synthesis of PGE2.

5. The fact that during pregnancy the increase in the vasodilator PGE2 is not counteracted by an increase in the constrictor TXA2 must be taken into account when explaining the glomerular hyperfiltration which characterizes the pregnant state.

6. The increase of PGE2 synthesis during dioestrus should be considered in studies of PG synthesis using virgin rats as controls.

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