Cardiac interstitial fibrillar collagen accumulation, such as that associated with chronic pressure overload (PO), has been shown to impair left ventricular diastolic function. Therefore, insight into cellular mechanisms that mediate excessive collagen deposition in the myocardium is pivotal to this important area of research. Collagen is secreted as a soluble procollagen molecule with NH(2)- and COOH (C)-terminal propeptides. Cleavage of these propeptides is required for collagen incorporation to insoluble collagen fibrils. The C-procollagen proteinase, bone morphogenic protein 1, cleaves the C-propeptide of procollagen. Procollagen C-endopeptidase enhancer (PCOLCE) 2, an enhancer of bone morphogenic protein-1 activity in vitro, is expressed at high levels in the myocardium. However, whether the absence of PCOLCE2 affects collagen content at baseline or after PO induced by transverse aortic constriction (TAC) has never been examined. Accordingly, in vivo procollagen processing and deposition were examined in wild-type (WT) and PCOLCE2-null mice. No significant differences in collagen content or myocardial stiffness were detected in non-TAC (control) PCOLCE2-null versus WT mice. After TAC-induced PO, PCOLCE2-null hearts demonstrated a lesser collagen content (PCOLCE2-null TAC collagen volume fraction, 0.41% ± 0.07 vs. WT TAC, 1.2% ± 0.3) and lower muscle stiffness compared with WT PO hearts [PCOLCE2-null myocardial stiffness (β), 0.041 ± 0.002 vs. WT myocardial stiffness, 0.065 ± 0.001]. In addition, in vitro, PCOLCE2-null cardiac fibroblasts exhibited reductions in efficiency of C-propeptide cleavage, as demonstrated by increases in procollagen α1(I) and decreased levels of processed collagen α1(I) versus WT cardiac fibroblasts. Hence, PCOLCE2 is required for efficient procollagen processing and deposition of fibrillar collagen in the PO myocardium. These results support a critical role for procollagen processing in the regulation of collagen deposition in the heart.