This study presents relationships between peripheral progesterone and Insulin-like Growth Factor-1 (IGF-1) concentrations during the early luteal phase in sows. Data were derived from three experiments, one with primiparous weaned sows (n = 21) and two with multiparous sows that either ovulated during lactation (n = 23) or after weaning (n = 12). The sows that ovulated during lactation did so due to an intermittent suckling regime (inhibition of suckling for 12 h each day from day 14 of lactation) or due to treatment with PG600. IGF-1 concentrations varied considerably among experiments, and were the lowest in the multiparous sows, regardless of whether they were weaned or lactating: 68 +/- 5 and 85 +/- 8 ng/ml in the two experiments with multiparous sows vs 188 +/- 15 ng/ml in the primiparous sows. Progesterone concentrations were lowest for the lactating sows. Overall, the increase in progesterone during the early luteal phase was strongly correlated with IGF-1 concentrations (r = 0.7). However, the correlation was low in multiparous lactating sows (r = 0.28; p < 0.10) and nonsignificant in multiparous weaned sows (r = 0). The weaned multiparous sows had IGF-1 levels comparable to lactating multiparous sows, but higher progesterone levels. In conclusion, these data show a positive relationship between peripheral IGF-1 and progesterone concentrations in vivo during the early luteal phase. In lactating sows, IGF-1 concentrations are probably a limiting factor for progesterone secretion, although other factors may be involved.