The purpose of this study was to determine if locally applied insulin has a dose-responsive effect on posterolateral lumbar fusion. Adult male New Zealand White rabbits underwent posterolateral inter-transverse spinal fusions at L5-6 using sub-optimal amounts of autograft. Fusion sites were treated with collagen sponge soaked in saline (control, n=11), or with insulin at low (5 or 10 units, n=13), mid (20 units, n=11), and high (40 units, n=11) doses. Rabbits were euthanized at 6 weeks. The L5-L6 spine segment underwent manual palpation and radiographic evaluation performed by two fellowship trained spine surgeons blinded to treatment. Differences between groups were evaluated by ANOVA on Ranks followed by post hoc Dunn’s tests. 43 rabbits were euthanized at the planned 6 week endpoint, while 3 died or were euthanized prior to the endpoint. Radiographic evaluation found bilateral solid fusion in 10%, 31%, 60%, and 60% of the rabbits from the control and low, mid, and high dose insulin treated groups respectively (p<0.05). As per manual palpation, 7 of 10 rabbits in the mid dose insulin group were fused as compared to 1 of 10 rabbits in the control group (p<0.05). This study demonstrates that insulin enhanced the effectiveness of autograft to increase fusion success in the rabbit posterolateral inter-transverse spinal fusion (PLF) model. The study indicates that insulin or insulin-mimetic compounds can be used to promote bone regeneration. This article is protected by copyright. All rights reserved.