In a recent study, we showed that a stretch/release cycle of ~20-25% L_o leads to a marked increase in intracellular calcium levels [Ca²⁺]_i in relaxed neonatal, but not in adult, rat muscle fibre bundles; we proposed that this increase may play a role in post-natal muscle differentiation (Mutungi, et al., 2003). In cultured skeletal myocytes an increase in [Ca²⁺]_i has been found to up-regulate slow-fibre type specific gene promoters via the activation of the cyclosporin-sensitive, calcium-calmodulin regulated serine/threonine phosphatase, calcineurin and these effects are partly mediated by members of the nuclear factor of activated T-cells (NFAT) family (Chin et al., 1998). However, little is known about the expression of members of the NFAT family during post-natal differentiation. Therefore, the aim of this study was to investigate the expression of two members of this family, NFATc1 and NFATc2, in fast and slow muscles of both neonatal (1-21 days old) and adult (>30 days old) rats. The rats were humanely killed with an overdose of sodium pentobarbitone injected intra-peritoneal. Immediately after death, the extensor digitorum longus (edl, mainly a fast muscle) and soleus (a predominantly a slow muscle) were carefully dissected. They were mounted onto cryostat chunks (half perpendicularly and the other half horizontally) and rapidly frozen in liquid nitrogen. 10-15μm thick serial sections were cut. One section from each group was stained for alkaline myosin ATPase activity (for fibre type identification) and the others for NFATc1 and c2 expression using monoclonal antibodies. Non-specific antibody binding was prevented by incubating the sections, for 30 minutes, in a blocking buffer (PBS containing 2% horse serum). In both the fast and slow muscles, no NFATc1 expression was seen in the cytoplasm of fibres of rats <10 days old (n=4) but they all stained strongly for NFATc2. Thereafter, the expression of NFATc1 in the cytoplasm of the fibres progressively increased whereas that of NFATc2 decreased with age. Thus, in adult rats 5±0.8% of the fibres in the soleus (n=5rats) and 39.8±4.4% of the fibres in the edl (n=5 rats) expressed NFATc1 in their cytoplasm. In the same muscles 42.3±0.9% of the fibres in the soleus and 2±0.1of the edl fibres expressed NFATc1 in their nuclei. On the other hand, NFATc2 was found to be mainly expressed in the nuclei of the adult fibres with 46.4±2% of edl fibres and 57.6±0.9% of soleus showing the presence of this isoform in their nuclei. These results suggest an age and fibre type dependent expression of the two NFAT isoforms investigated. However, their physiological role in these fibres remains uncertain. All values are mean ±S.D.