Myocardial hypertrophy is characterized by abnormal intracellular Ca²⁺ handling and decreased contractile performance. Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylates numerous Ca²⁺ handling proteins and thus can regulate intracellular Ca²⁺ homeostasis directly. We therefore investigated whether differential expression of CaMKII isoforms occurs with cardiac hypertrophy which might promote an abnormal intracellular Ca²⁺ homeostasis. We further investigated the potential influence of angiotensin (Ang) II on CaMKII expression levels. Hearts from adult Spontaneously Hypertensive Rats (SHR) and hearts from two transgenic rat models with Ang II-dependent hypertension were studied. The expression of the cardiac CaMKII isoforms δ₂, δ₃, δ₄ and δ₉ was determined by RT-PCR and immunoblot methods. Rats transgenic for the mouse Ren-2 gene (mrTGR), SHR and controls were studied at the age of 6 months and rats transgenic for the human renin-angiotensin system (hrTGR) from postnatal day 1 to week 8. SHR and mrTGR had an increased heart/body weight ratio (26 and 25%) compared with controls (p < 0.05). SHR hearts showed significantly increased mRNA levels of δ₄ and δ₉ (p < 0.05) with no change for δ₂ and δ₃. mrTGR hearts had a significantly increased δ₄ and a significantly decreased δ₃ transcript level (p < 0.05) with no change for δ₂ and δ₉. hrTGR hearts developed severe hypertrophy (42%) after postnatal day 14. The neonatal δ₂, δ₃ and δ₄ isoform expression levels were higher (30–100%) compared with SD controls. The levels decreased with increasing age and equalized to controls at week 8, except for δ₄ which started to increase after week 4 (p < 0.05). CaMKIIδ protein levels of all cardiac hypertrophy models were increased in sarcoplasmic reticulum preparations (50–120%) compared with controls (p < 0.05) while the cytosolic levels remained unchanged. Thus, CaMKIIδ isoforms are differentially expressed in cardiac hypertrophy. The fetal δ₄ isoform was constantly expressed. CaMKIIδ adopts the fetal phenotype independent of the type of hypertrophic stimulus. The observed alterations of CaMKIIδ isoform patterns may affect intracellular Ca²⁺ homeostasis and thus contribute to the abnormal contractile phenotype of cardiac hypertrophy.