Introduction Heat stress alters the follicular development pattern in cattle. Exposure of cows to heat stress led to a reduction in the size of the dominant follicles of the ﬁrst and second follicular wave of the oestrous cycle (Badinga et al., 1993; Wilson et al., 1998a,b). Depression of follicular dominance by heat stress was indicated by: the absence of a decrease in medium sized follicles during the ﬁrst follicular wave or during the follicular phase of the oestrous cycle; a large size and a slow decrease in the size of the second largest follicle; an increase in the number of large follicles during the ﬁrst follicular wave; and an early emergence of the preovulatory follicle (Badinga et al., 1993; Wolfenson et al., 1995; Wilson et al., 1998b). These studies examined the responses to heat exposure during days 1–8 of the oestrous cycle (Badinga et al., 1993), during days 11–21 of the oestrous cycle (Trout et al., 1998; Wilson et al., 1998a,b) or during a hormonally programmed oestrous cycle (Wolfenson et al., 1995). However, the effect of heat stress on follicular dynamics during a complete unprogrammed oestrous cycle, in particular, the turnover of the ﬁrst and the second follicular waves has not been investigated. Furthermore, FSH secretion and its association with follicular dynamics in heat-stressed Journal of Reproduction and Fertility (2000) 120, 83–90 Immediate and delayed effects of heat stress on follicular development and its association with plasma FSH and inhibin concentration in cows Z. Roth 1 , R. Meidan 1 , R. Braw-Tal 2 and D. Wolfenson 1 * 1 Department of Animal Science, Faculty of Agriculture, Food and Environmental Quality Sciences, The Hebrew University of Jerusalem, Rehovot 76100, Israel; and 2 Institute of Animal Science, Agricultural Research Organization, The Volcani Center, Bet Dagan 50250, Israel The aim of this study was to characterize the immediate effects of heat stress on plasma FSH and inhibin concentrations, and its involvement in follicular dynamics during a complete oestrous cycle, and to examine a possible delayed effect of heat stress on follicular development. Holstein dairy cows were oestrous synchronized and randomly assigned to either cooled (n = 7) or heat-stressed (n = 6) treatment groups. During a complete oestrous cycle, control cows, which were cooled, maintained normothermia, whereas heat-stressed cows, which were exposed to direct solar radiation, developed hyperthermia. At the end of this oestrous cycle (treated cycle), both groups were cooled and maintained normothermia for the ﬁrst 10 days of the subsequent oestrous cycle. Throughout this period, follicular development was examined by ultrasonography, and plasma samples were collected. During the second follicular wave of the treated oestrous cycle, a signiﬁcantly larger cohort of medium sized follicles (6–9 mm) was found in heat-stressed cows than in cooled cows (P< 0.05). The enhanced growth of follicles in this wave in heat-stressed cows was associated with a higher plasma FSH increase which lasted 4 more days (days 8–13 of the oestrous cycle; P< 0.05), and coincided with a decrease in the plasma concentration of immunoreactive inhibin (days 5–18 of the oestrous cycle; P< 0.05). During the follicular phase (days 17–20 of the treated cycle), heat-stressed cows showed an increase in the number of large follicles ( 10 mm), and the preovulatory plasma FSH surge was signiﬁcantly higher in heat- stressed cows than in cooled cows (P< 0.01). The effect of heat stress was also observed during the ﬁrst follicular wave of the subsequent cycle: the postovulatory plasma FSH concentration was higher (P< 0.01), but fewer medium follicles developed, and the ﬁrst follicular wave decreased at a slower rate in previously heat-stressed cows than in cooled cows (0.40 and 0.71 follicles per day, respectively). This study shows both immediate and delayed effects of heat stress on follicular dynamics, which were associated with high FSH and low inhibin concentrations in plasma. These alterations may have physiological signiﬁcance that could be associated with low fertility of cattle during the summer and autumn. © 2000 Journals of Reproduction and Fertility Ltd 0022–4251/2000 *Correspondence. Received 7 January 2000.