In laying hens, the primary sources of progesterone (P4) are the granulosa cells of the 4-5 largest follicles in the hierarchy of the ovary. During forced moulting, cessation of egg production occurs with ovarian atrophy and a decrease in P4 as well as in luteinizing hormone. While it is reported that follicular atresia as well as the P4 decrease are the result of a reduced luteinizing hormone (LH) stimulation, P4 decrease occurred already during the second or third day after food withdrawal while atresia occurred only several days later. Since P4 decrease at the beginning of moult induction cannot simply be explained by the lack of large follicles the aim of this study was to determine whether the rapid decrease in P4 after food withdrawal is only related to a decreased hypophyseal stimulus and/or a concomitant change in follicular sensitivity as well. Granulosa cells from follicles F1 to F4 were collected according to the method of Gilbert et al. (1977). Cell number was estimated with a haemocytometer and viability by trypan blue exclusion technique. Cell suspension was diluted to 10(5) cells ml-1 in M199 and incubated for 0.5, 1, 2, 4, 8 and 24 h with or without oLH, and P4 secretion in the medium was assayed by RIA. Short-term starvation increased P4 response in vitro to oLH indicating a higher sensitivity of granulosa cells of follicles from all classes to oLH. In an additional experiment, in vivo injections of different doses of LH also showed an enhanced increase in P4 the first days of food restriction. Therefore, the rapid decrease in P4 after feed withdrawal is not due to a decreased follicular sensitivity but to a decrease in stimulatory factor(s).