1. This study analyses whole blood in acutely unclipped one-kidney, one-clip (1K,1C) hypertensive rats for the presence of platelet-activating factor (PAF), a potent vasodilator and a putative mediator of the rapid blood pressure (BP) fall seen after unclipping.

2. Hypertensive 1K,1C rats were anaesthetized and a carotid and jugular cannula were inserted for BP measurement and anaesthetic infusion respectively. After a stable level of anaesthesia was attained, the constrictive clip was removed and BP was recorded for 30 min.

3. Blood was drawn from the aorta directly into ice-cold acetone. The extract was analysed for PAF by a bioassay using 5-hydroxy-[14C]tryptamine-labelled platelets.

4. Rats which showed a BP fall had elevated levels of PAF [55 ± 6 (sem) pg/ml] when compared with those with no fall in BP [26 ± 7 (sem) pg/ml] (P < 0.01).

5. This supports the hypothesis that activation of PAF biosynthesis may be a mechanism contributing to the fall in BP seen after unclipping the 1K,1C hypertensive rat.

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