The aim of this study was to investigate the role of FSH in the control of ovulation rate by the Booroola gene. Three Booroola genotypes (FecBFecB, FecBFec+ and Fec+Fec+) of the F2 population, from a cross between Booroola Merino and Scottish Blackface, and two Booroola genotypes (FecBFec+ and Fec+Fec+; 25% Booroola Merino and 75% Scottish Blackface), from the backcross of FecBFec+ sires to Scottish Blackface ewes, were compared. During seasonal anoestrus significant differences (P < 0.05) in hCG-stimulated ovulation rates were obtained between FecBFecB and Fec+Fec+ ewes from the F2 population, and FecBFec+ ewes were intermediate. No significant difference in hCG-stimulated ovulation rate was observed in the backcross population between FecBFec+ ewes and Fec+Fec+ ewes. There were no significant differences between genotypes in mean serum FSH concentrations during seasonal anoestrus in either backcross of F2 population. During the breeding season, two separate experiments confirmed the expected ovulation rate differences between genotypes (FecBFecB > FecBFec+ > Fec+Fec+). In both experiments, mean peripheral FSH concentrations in the F2 population were similar in FecBFec+ and Fec+Fec+ ewes, but were significantly higher (P < 0.05) in FecBFecB ewes. In the backcross population, mean peripheral FSH concentrations during the oestrous cycle were not significantly different between FecBFec+ and Fec+Fec+ ewes, despite significant differences in ovulation rate. Ovariectomy during the breeding season resulted in significantly higher (P < 0.001) mean peripheral FSH concentrations in all three genotypes. After ovariectomy, mean FSH concentrations between FecBFec+ and Fec+Fec+ ewes, form both backcross and F2 populations, were not significantly different.(ABSTRACT TRUNCATED AT 250 WORDS)