1. Using 12 castrated Large White pigs, the way in which the size and number of recognizable fat cells (i.e. > 6·0 μm) increases during growth from 26 to 109 kg live weight, and the relationships between fat cellularity and body fat content were determined.
2. Fat biopsy samples were taken at 87, 145 and 188 days of age from the shoulder region. This region grew rapidly in volume but was early maturing since its relative growth during the period of the experiment was low compared with other regions, particularly the mid-back to loin region.
3. The growth of fat that occurred during the experiment was due to increases in both the size and number of recognizable cells. There was no indication that the number of cells had become constant even at 188 days of age.
4. Subcutaneous or total fat expressed as a percentage of total dissectible tissue weight was related quite closely (r = 0·6 to 0·7, residual s.d. about 8% of mean fat percentage) to average fat cell size measured at 145 and 188 days. The number of cells at these times, defined as the number in a cylinder whose depth was known and whose cross-sectional area was 1 mm2 on day 1, was not significantly correlated with whole body fatness.
5. Appetite was strongly linked to the rate of fat deposition and to the weight of fat at 188 days, but not to fat expressed in percentage terms. This was, however, closely related to the corrected food conversion ratio (r = 0·90, residual s.d. 2% of mean total fat percentage).
6. Fat thickness measured by ultrasonics before slaughter was no better as a predictor of percentage fat than average fat cell size.