Mechanical compression and distraction forces are known to modulate growth in vertebral growth plates, and are implicated in the progression of scoliosis. The amount of cellular enlargement is thought to be a key variable correlating with differing growth at different anatomical sites. This work tested the hypothesis that growth differences produced by mechanical loading are associated with alterations in the amount of cell height increase in the growth direction. Compression force (3 animals) or distraction force (4 animals) of 60% bodyweight was maintained on a caudal vertebra of growing rats with an external apparatus attached to vertebrae adjacent to the experimental level via transfixing pins. Growth of loaded and control vertebrae was measured from weekly radiographs. After four weeks, animals were euthanized and the loaded vertebrae and adjacent control vertebrae were excised for histological sections from which the following were measured: height of the hypertrophic zone (the average separation between boundaries of the zone); Mean Hypertrophic chondrocytic height; slope of the linear regression of cell height vs. position in the zone, (the amount of cell enlargement). Distracted and compressed vertebra growth averaged 110% and 53% of control levels. The percentage changes in the measures of chondrocyte dimensions, relative to control values, were smaller than the percentage changes in growth velocity. Also, the effects were of smaller magnitude for distracted vertebrae than for compressed vertebrae. Growth plate zonal height, slope and mean cell height all correlated significantly (p<0.05) with growth rate (expressed as a percentage of control) (Pearson r = 0.69, 0.37 and 0.41 respectively. Thus mechanical loading of tail vertebrae modulated their growth rate, that in turn correlated with changes in the amount of hypertrophic chondrocyte height increase. The effects for compression were greater than for distraction.