The success of sheep breeding programmes involving sire reference schemes depends on effective oestrous synchronisation programmes that provide optimum pregnancy rates following fixed-time insemination (McKelvey & Simm, 1995). Effective oestrous synchronisation is characterised by low progesterone concentrations at the time of insemination. However in pedigree Suffolk ewes inseminated at a synchronised oestrus in early August, Dingwall et al (1995) reported that a significant number of ewes in some flocks had elevated progesterone concentrations at the time of insemination and, as a consequence, flock fertility was adversely affected. It is hypothesised that high progesterone reflects aberrant luteal activity arising from spontaneous ovulations following the preceding lambing. The present study sought to identify the incidence of aberrant luteal activity in Suffolk ewes during seasonal anoestrus and to study the effect on conception of interventional treatment of these animals prior to artificial insemination.