AbstractThe effect of amrinone on platelet production was differentiated from that of a known bone‐marrow cytotoxic agent (cyclophosphamide) and anti‐platelet serum (APS). The rate of platelet production has been observed over a 4‐day period in the Gunn rat using [75Se]selenomethionine cohort labelling of platelets following administration of either amrinone, 160 mg kg−1day−1, cyclophosphamide, 30 mg kg−1day−1or APS, 0.75 ml. Platelet numbers were reduced by amrinone, cyclophosphamide and APS. The rate of platelet production was increased following APS and suppressed by cyclophosphamide, but the rate of platelet production when expressed as the selenomethionine incorporation in counts per minute (cpm) per 108platelets appeared to be increased in amrinone‐treated animals. When these values are expressed as radioactivity per unit platelet volume the difference between the control and the amrinone‐treated group was reduced but the difference between the control, APS‐ and cyclophosphamide‐treated groups remained unchanged. It is concluded that in the Gunn rat amrinone affects platelet production by producing fe