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‐PGF1α levels by 31%, only nonsignificantly. Urine excretion of PGI‐M and TxB2 did not change significantly after CsA infusion. Tac decreased TxB2 in the COX‐1 ex vivo assay by 30% (P = 0.03), while no changes were seen in urinary levels of PGI‐M or TxB2. Urinary TxB2 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.