Poor conception rates in highly productive lactating cattle is especially prevalent in large, intensively-managed commercial herds. One of the causative factors is sub-optimal pre-implantation embryonic development which appears to result from inadequate circulating concentrations of progesterone. In the present study, the efficacy of very modest progesterone supplementation, between Days 3.5 and 10 post-AI, on pregnancy rates was determined in a commercial herd where bovine somatotropin (bST) was used as a management tool. All lactating cattle that were deemed to be in estrus and inseminated over a 4-week period were randomly assigned to either a control group (no treatment) or CIDR-1.9g (previously used for estrous synchronization) treatment from Day 3.5 to Day 10 post-AI. Milk samples were collected four times: on the day of AI, at Day 2 or 3, at Day 4 and at Day 22 post-AI and were analyzed for progesterone content. Data from a total of 130 breedings were used in the final analysis. The CIDR treatment increased circulating concentrations of progesterone in treated animals over those of control animals on Day 4 by 0.7ng/ml (P