Twenty first-liter sows were used to investigate the relationship between plasma lactose concentrations following weaning and milk production, total milk content of lactose, post-weaning plasma oestradiol-17beta (E2) concentration and weaning to mating interval. Milk production was estimated from 6 out of 8 hourly successive "weighing-suckling-weighing" of piglets on day 21 of a 28-day lactation. Sows were cannulated in the jugular vein and sampled at 8-h intervals from 2 h to 66 h after weaning. Plasma lactose concentrations (means +/- SE) after weaning increased from 52 +/- 4 microM at the beginning of sampling to a peak value of 183 +/- 23 microM 40 h later and then decreased to 91 +/- 11 microM 66 h after weaning (P less than 0.001). Milk production on day 21 (7.5 +/- 0.3 kg) and the corresponding milk content of lactose (400.5 +/- 0.2 g) were not related (P greater than 0.10) to area under the curve, timing, amplitude and spreading of the lactose peak in plasma. Similarly, none of the characteristics of the lactose peak were related (P greater than 0.10) to weaning to mating interval. There was a linear increase (P less than 0.0001) in mean plasma E2 concentrations (means +/- SE) from 5.6 +/- 0.3 pg/ml, 2 h after weaning, to 12.9 +/- 1.2 pg/ml at the end of the sampling period. On a within-sow basis, there was a correlation (r = 0.28; P less than 0.01) between post-weaning plasma lactose and E2 concentrations. Results showed the existence of the post-weaning peak of plasma lactose in sows.(ABSTRACT TRUNCATED AT 250 WORDS)