Earlier reports indicate that lambs born to ewes that experienced sub-clinical cobalt/vitamin B12 deficiency during pregnancy are slow to stand and suck and have increased morbidity and mortality. These problems were not prevented by cobalt supplementation during mid to late pregnancy (Fisher & MacPherson, 1991), suggesting that they may originate during early development. The aim of this study was to distinguish between the effects of cobalt/vitamin B12 status of ova and of the ewe uterine environment on lamb behaviour using reciprocal embryo transfer. Scottish Blackface ewes from cobalt-deficient farms were either untreated (Co-, n=82) or were given an intra-ruminal cobalt-containing bolus ~30 days before embryo transfer (Co+, n=82). Day 6 embryos were recovered from 33 superovulated Co- or Co+ ewes and transferred singly to Co- or Co+ recipient ewes. Lamb behaviour was recorded by focal observation at birth and during the first 3 days of life by scan sampling at 2-hourly intervals. Data were analysed by linear mixed models using natural log transformations of behaviour data that were not normally distributed. Circulating concentrations of vitamin B12 on the day of ovum recovery were higher in Co+ than Co- donors (P<0.001). There was no effect of treatment on the proportion of recipients that became pregnant. The number of lambs studied in each of the 4 treatment groups was between 10 and 14. Concentrations of vitamin B12 were lower in Co- compared to Co+ ewes during pregnancy (P<0.001) and in lambs born to Co- compared to Co+ ewes at birth (Co- = 543 pmol/l, Co+=1805 pmol/l s.e.d.=92.1, P<0.001). There was no effect of donor or recipient cobalt/vitamin B12 status on lamb birth weight. There were no consistent effects of donor or recipient cobalt/vitamin B12 status on lamb behaviours immediately after birth. However, lambs born to Co+ recipient ewes were slower to get to their knees and to attempt to stand than lambs from Co- recipients (median time (mins with interquartile ranges): Co+ recipient=12.23 (6.35-13.83), Co-recipient=8.14 (4.98-11.99), P<0.05). Donor and recipient cobalt/vitamin B12 status had no effect on mean ewe-lamb distance during the first 3 days of life, or on any aspect of ewe behaviour. Lambs from Co+ donors were more active than lambs from Co- donors; they stood more frequently (% observations active: Co+ donor = 29.63, Co- donor = 21.99, s.e.d. = 2.43, P<0.005), were more frequently observed interacting with their mother (% observations: Co+ donor = 6.30, Co-donor = 3.11, s.e.d.=1.22, P<0.01) and spent more time exploring their environment (% observations: Co+ donor = 16.55, Co- donor = 6.85, s.e.d.=2.30, P<0.001). These data indicate that nutrient status prior to mating and during the early cleavage stages of embryo development, as well as during later pregnancy, affects lamb behaviours. Acknowledgements: This work was funded by SEERADReference 1 : Fisher GEJ & MacPherson A (1991). Res Vet Sci 50, 319-327.