Background: Morphine concentrations(MOR) vary widely and measurements often take days. Most studies look at various doses vs concentration, but not at dose or concentration-effect relationship .Aim: To study whether MOR concentrations measured within hours by Fluorescention Polarisation Immuno Assay(FPIA) can be useful to adjust dosage in preterm infants and to determine a concentration-effect relationship in the first days after birth. Subjects: 10 preterm, ventilated infants, gestational age(mean±SD) 28.9±2.4 wks, birthweight 1257±572 gms. Intervention: MOR 0.1 mg loading dose on day 1, followed by 10 μg/kg/hr continuously. Bloodsamples were taken on day 2, 3 and 5. Neonatal infant pain score(NIPS) was assessed by trained nurses, every 4 hours and once daily during tracheal suction. Results: Mean MOR concentrations on day 2(n=10) were 91±30 μg/l(range 40-161), on day 3(n=7) 81±32(42-146), and on day 5 (n=4) 112±73(52-233). There was no relation between NIPS and MOR concentration, but all infants had scores from 0.3 during MOR, mean 0.7±0.89, as opposed to scores from 3.6, mean 4.1±1.9 before MOR(P<0.001). Scores during suction increased to mean of 4±1.9, range 0.6 and were higher than before suction(P<0.001).Conclusion: MOR in this dosage seems to give good analgesia as assessed by NIPS, concentration can be assessed by FPIA. There is no correlation between NIPS and MOR concentration. NIPS is significantly higher before MOR and during suction.