Long bone lengthening occurs at the growth plate (GP) by well-regulated chondrocyte proliferation, hypertrophy and terminal matrix deposition (1,2). GP chondrocyte (GPC) hypertrophy contributes >60% to bone elongation (1), however the mechanism is poorly understood although membrane transporters involved in the control of cell volume and pH are likely to be important (3). Here, we have examined changes to levels of the Na+-independent Cl-/HCO3- anion exchanger 2 (AE2) and the Na+/H+ exchanger 1 (NHE1) along the growth plate using fluorescence immunohistochemistry (FI). GP from proximal tibiae of six humanely-killed 7-day rat pups were dissected, sagitally bisected and fixed in 4% paraformaldehyde. Tissue sections were prepared for FI and probed with 1o Abs for AE2 or NHE1 (Abcam, Cambridge) with fluorescently-tagged 2o Abs (Invitrogen, UK) visualized using confocal scanning laser microscopy (CLSM, Zeiss, Germany). Distribution of AE2 and NHE1 associated with chondrocytes at various stages of the differentiation process along the GP was quantified using imaging software (ImageJ, Maryland, USA). AE2 showed a gradual increase in fluorescence labelling from the early proliferative zone to the early stage of hypertrophy where it reached a significantly higher level (n=3; unpaired Student’s t-test; P=0.01). Labelling then declined dramatically towards the terminal end of the GP (P=0.002). Labelling of NHE1 demonstrated an apparent initial slight increase from the early proliferative zone, then remained relatively constant through the proliferative and early hypertrophic zone until decreasing towards the end hypertrophic stage (P=0.008). Increased levels of AE2 associated with GPC appeared to correspond with early chondrocyte enlargement whereas levels of NHE1 were relatively constant throughout the proliferative and early hypertrophic zone and support the role for this transporter in the maintenance of optimal cellular pH. These results suggest complex changes to levels of these transporters along the growth plate which might be associated with GPC hypertrophy and thus the rate of bone lengthening. Changes to levels of these transporters have also been associated with hypertrophy and cell survival in other cell types (4) raising the possibility that they play similar roles in the growth plate.