The calcineurin inhibitors (CNIs) cyclosporine (CsA) and tacrolimus (Tac) are implicated in post-transplant complications such as cardiovascular morbidity. Prostanoids are fatty acid-derived compounds essential for controlling cardiovascular homeostasis. We tested the hypothesis that CNIs suppress cyclooxygenase (COX)-2-derived prostacyclin (PGI) and increase thromboxane synthesis in humans. Ten healthy men underwent 5-h infusions of CsA, Tac, and saline in a randomized, double-blind, cross-over study. Blood and urine samples were collected before and after the infusion of each drug/saline, to measure PGI and thromboxane metabolites. CsA decreased whole-blood COX-2 activity by 39% (P = 0.05) and basal plasma 6-keto-PGF(1α) levels by 31%, only nonsignificantly. Urine excretion of PGI-M and TxB(2) did not change significantly after CsA infusion. Tac decreased TxB(2) in the COX-1 ex vivo assay by 30% (P = 0.03), while no changes were seen in urinary levels of PGI-M or TxB(2) . Urinary TxB(2) excretion was 15% lower after saline infusion (P = 0.03). These within-treatment differences in prostanoid synthesis did not differ significantly between the treatments (anova). Mean blood levels of CNIs were 486 μg/l for CsA and 12.8 μg/l for Tac. Clinically relevant doses of CsA and Tac induce acute differential changes in prostanoid levels in healthy human subjects. CsA suppresses COX-2 activity, while Tac decreases platelet activity.