Western blotting was used to identify the array of protein kinase C (PKC) isozymes expressed in the early (Day 4) and midcycle (Day 10) bovine corpus luteum (CL). PCKα, βI, βII, ϵ, and μ isozymes were detected in total protein samples prepared from both Day-4 and Day-10 corpora lutea. In contrast, specific antibodies for PKCγ, η;, λ, and θ isozymes failed to detect protein bands in the luteal samples. PKCβII and ϵ isozymes were expressed differentially at these two developmental stages of the bovine CL. In the Day-4 luteal samples, PKCϵ was barely detectable; in contrast, in the Day-10 samples, the actin-corrected ratio for PKCϵ was 1.16 ± 0.13. This ratio was higher than the detected ratio for PKCβI and μ at this developmental phase of the CL (P < 0.01), but it was comparable with the ratio detected for the PCKα and βII. The amount of PKCβII was, although not as dramatic, also greater in the Day-10 CL (actin-corrected ratio was 0.85 ± 0.2) than in the Day-4 CL (0.35 ± 0.09 [P < 0.01]). The actin-corrected ratios for all other PKC isozymes, α (Day 4 = 0.93 ± 0.16, Day 10 = 0.97 ± 0.09), βI (Day 4 = 0.54 ± 0.073, Day 10 = 0.48 ± 0.74), and μ (Day 4 = 0.21 ± 0.042, Day 10 = 0.21 ± 0.38) were not different at these 2 days of the cycle. An experiment was designed to test whether activation of specific isozymes differed between CL that do or do not regress in response to PGF_2α. Bovine CL from Day 4 and Day 10 of the estrous cycle were collected and 1 mm CL fragments were treated in vitro for 0, 2.5, 5, 10 or 20 min with PGF_2α (0.1, 1.0, and 10 nM) or minimal essential medium-Hepes vehicle. Translocation of PKC from cytoplasm to membrane fraction was used as indication of PKC activation by PGF_2α. Evidence for PKC activation was observed in both Day-4 and Day-10 luteal samples treated with 10 nM PGF_2α. Therefore, if PKC, an intracellular mediator associated with the luteal PGF_2α receptor, contributes to the lesser sensitivity of the Day-4 CL, it is likely due to the differential expression of the ϵ and βII isozymes of PKC at this stage and not due to an inability of the PGF_2α receptor to activate the isozymes expressed in the early CL.