Two experiments were carried out in which follicular aromatase activity was compared between Meishan and Large-White hybrid gilts. In Expt 1, preovulatory follicles (n = 10 largest per animal) were recovered from Meishan and Large-White hybrid gilts (n = 5 per breed) on the day before predicted onset of behavioural oestrus, and the granulosa cells and theca tissue incubated to determine aromatase activity. Follicles recovered from Meishan pigs were smaller (P < 0.01) and contained fewer

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... a cells (P < 0.05), but follicular oestradiol content of the breeds was similar (P> 0.1). Aromatase activity was higher in the theca tissue (P < 0.05) and tended to be higher in the granulosa cells recovered from Meishan follicles (P = 0.065). In Expt 2, granulosa cell aromatase activity was investigated during the early follicular phase (estimated day 16 of cycle) in Meishan and Large-White hybrid gilts (n = 6 and 5, respectively). The number of follicles \ m=ge\ 1 mm diameter recovered per animal was 171 for both breeds (P>0.1), whereas the number of follicles \m=ge\2 mm diameter was 65 and 101 (P < 0.05) from Meishan and Large-White hybrid gilts, respectively. The mean diameter of all follicles recovered was smaller in the Meishan gilts (P< 0.001). Overall, neither the number of granulosa cells per follicle, as indicated by DNA estimation, nor the oestradiol content differed between the breeds at this time (P>0.1). Mean aromatase activity was assessed in granulosa cells recovered from alternate follicles \m=ge\3mm diameter and was higher in the Meishan gilts when expressed per \g=m\g DNA (P \m=l e\0.025). As follicle diameter increased, the difference in aromatase activity between the breeds became more pronounced (P < 0.05). These results indicate that Meishan follicles have higher aromatase activity in the granulosa and theca cells during both the early and late follicular phase, compared with Large-White hybrid follicles. Whether this has a role in ensuring the prolificacy of the Meishan pig requires further investigation.