Follicular growth rates were studied in 5 Hereford-Holstein cross heifers on Day 14 of the oestrous cycle. The granulosa cell mitotic index (MI) was measured in non-atretic antral follicles of various diameters (0.13-8.57 mm) from Bouin-fixed ovaries collected before (199, control) and 2 h after colchicine treatment (189, treated). In control ovaries, follicles of 0.68-1.52 mm had a higher MI than those of other size classes (P less than 0.05). In colchicine-treated ovaries, the MI of follicles ranging from 0.68 to 8.57 mm increased more than that of other sized follicles, so that the mitotic time was shorter (0.78 h vs 1.32 h) in medium and large sized follicles (0.68-8.57 mm) than in smaller follicles (0.13-0.67 mm). Calculations based on the number of granulosa cells in follicles of various classes and from the time required to double the number of cells within a follicle indicate that a follicle takes 27 days to grow from 0.13 to 0.67 mm, 6.8 days from 0.68 to 3.67 mm and 7.8 days from 3.68 to 8.56 mm, indicating that growth rates varied with the size of the follicle. A period equivalent to 2 oestrous cycles would therefore be required for a follicle to grow through the antral phase, i.e. from 0.13 mm to preovulatory size. Increased MI, decreased mitotic time and increased atresia found in follicles larger than 0.68 mm could indicate a change in the follicular metabolism during its maturation.