This study assesses the relatively high incidence of the expression of paralogs of several pseudogenes within the cascade of expression of functional genes in the rat ovary in response to an ovulation-stimulating dose of gonadotropin. Immature Wistar rats were primed with 10 IU equine chorionic gonadotropin subcutaneously, and 48 h later the 12-h ovulatory process was initiated by 10 IU hCG subcutaneously. Ovarian RNA was extracted at 0, 2, 4, 8, 12, and 24 h after injecting the animals with hCG. The RNA extracts were used for RT-PCR differential display to detect gene expression in the ovarian tissue. Sequence analyses of differentially expressed cDNAs revealed that approximately 27% (i.e. 22/82 clones) of the transcripts were fragments of paralogs of known pseudogenes. Out of the 22 clones reported here, 12 have high sequence similarity to the cytochrome P450 pseudogene Cyp21a1-ps, and 5 have high sequence similarity to both the Cyp21a1-ps and the aldo-keto reductase gene Akr1c6. The remaining five clones were paralogs of the endogenous retrovirus SC1 that has heavily infested the rat genome. Northern analyses reveal that peak expression of all the 22 paralogs occurs at 4-8 h into the ovulatory process. In situ hybridization shows that expression of these pseudogenes is primarily in the granulosa layer of ovulatory follicles. In summary, the results reveal that ovarian expression of Cyp21a1-ps- and SC1-like pseudogenes occurs concurrently with the ovulatory process.