During the previous decade several studies focused on postpartum treatment with prostaglandin for improvement of reproductive performance in sows. The aim of the study was to investigate the effect of administration of a prostaglandin F(2 alpha) (PGF(2 alpha)) analogue in sows within 24-48 h after farrowing on sow and litter performance. In five commercial farms, the sows were randomly assigned to either treatment A (2 ml cloprostenol, Planate) or treatment B (2 ml physiological saline solution, i.m.). Fifteen per cent of all sows were at random selected for progesterone analysis. Litter performance was assessed by measuring pre-weaning mortality and average daily weight gain (ADG). Sow performance was assessed by measuring weaning-to-oestrus interval (WOI), the percentage of sows returning to oestrus and litter size during subsequent farrowing. Administration of a PGF(2 alpha) analogue within 24-48 h postpartum had no effect on the rate of progesterone decline measured over 24 h compared with that of the controls. Litter performance and WOI were not affected by treatment. The subsequent litter size in sows of parity seven and more showed a significant difference of 1.98 piglets (p < 0.01) between both groups, to the benefit of the cloprostenol group. In conclusion, administration of a synthetic PGF(2 alpha) analogue, cloprostenol, within 24-48 h after farrowing improved litter size at next farrowing in older (>or=7 parity) sows.