Partial genomic clones for the 80 kDa subunits of rat calpains I and II have been isolated. Some exons have been located and sequenced, and used to synthesize RNA probes specific for each isoenzyme. The levels of total DNA, soluble protein, calpain II 80 kDa subunit and the mRNA for this subunit were measured in parallel in separate extracts of non-pregnant, pregnant and post-partum rat uteri. The amount of total DNA, expressed as mg/g wet wt. of tissue, was found to remain constant throughout this period, except for a slight rise during involution. Calpain I was present in all samples in very small amounts. The amounts of calpain II 80 kDa subunit (measured on immunoblots) and of its mRNA (measured by means of slot-blots) also did not vary, when expressed in terms of units per g wet wt., during the 10-fold growth of the uterus during pregnancy and its post-partum involution. It was concluded that expression of calpain II was constitutive in this normal tissue, which is undergoing rapid growth and involution under complex hormonal control.