Decreased diaphragmatic blood flow (Qdi) is associated with decreased contractility in adult animals. To see whether the decrease in Qdiassociated with a patent ductus arteriosus was associated with a decrease in diaphragmatic contractility Pdi, we prepared 11 near–term fetal lambs by infiltrating the ductus with formalin and placing a snare around it to regulate its patency. The lambs (with open chest) were delivered and mechanically ventilated, and the phrenic nerves and diaphragm were paced (using transvenous wires) at rates of 20/min, 60/min, and 100/min (Inspiratory time/total respiratory cycle time = 0.5) for 7–min contraction periods after 30-min recovery periods. Qdiwas measured with radiolabeled microspheres at the end of each contraction period. Diaphragmatic performance was determined by comparing Pdiat the start (Pdi–start) and end (Pdi–end) of the contraction period. When the ductus was closed, Qdiincreased 6.9-fold at 20/min and 9.8-fold at 100/min (compared with Qdiat rest). Pdi-end was less than Pdi–start at all contraction rates, but the reduction was significantly greater at 100/min (Pdi–end/Pdistart: 0.80 ± 0.10 at 20/min; 0.67 ± 0.17 at 100/min). Pdi–start also decreased with increasing rates of contraction. When the ductus was open, the left-to-right shunt was 64 ± 11% of left ventricular output.diin the unpaced diaphragm was significantly reduced (open 4.0 ± 3.8versusclosed, 7.4 ± 2.4 mL/min/100 g). However, with pacing,Qdiincreased with open ductus and so did not differ from Qdiwith closed ductus. Similarly, at each contraction rate, ductus patency did not alter diaphragmatic performance (Pdi–end/Pdi–start, openversusclosed: 20/min = 0.80 ± 0.15versus0.80 ± 0.10; 60/min = 0.76 ± 0.11versus0.73 ± 0.17; 100/min = 0.62 ± 0.14versus0.67 ± 0.17). We conclude that diaphragm performance decreases at faster contraction rates despite an increase in Qdi. Although a patent ductus arteriosus reducesdiin the noncontracting diaphragm, during contractionsQdican increase to levels achieved when the ductus is closed so that there is no change in diaphragmatic performance.