The present studies were designed to characterize LH release during pregnancy in the rat. Unanesthetized animals with jugular cannulae were bled for 3 h between 1000-1300 h on days 6-8, 14-16 or 22 of gestation (50 microliters whole blood/5 min). Plasma estradiol and progesterone values both increased from days 6-8 to days 14-16. However, while plasma estradiol levels increased further between days 14-16 and day 22, plasma P levels had declined 86%. The percent coefficients of variation obtained for alterations in blood LH levels at each stage of pregnancy were all significantly greater than intraassay variation, indicating that LH release was pulsatile at each stage. Although there were no significant differences in mean blood LH levels, pulse amplitude, or frequency between days 6-8 and 14-16, the individual patterns of LH release clearly varied between these 2 groups, and most notably within the 14-16 day group. Fifty-three percent (9 of 17) of the LH records in rats on days 14-16 were nonpulsatile compared to only 20% (3 of 15) on days 6-8. However, despite a trend toward an absence of pulsatile LH release on days 14-16, mean frequency at this time did not differ from days 6-8, since on days 14-16 the remaining 8 animals demonstrated 3.5 pulses/3 h, while on days 6-8 the other 12 rats averaged only 2.5 pulses/3 h. On day 22, there was a marked increase in mean blood LH levels compared with either days 6-8 or 14-16. This increase was due to an increase in mean LH pulse frequency. All 15 rats demonstrated pulsatile LH secretion, a significantly greater incidence of pulsatile LH release than on days 14-16 (100% vs. 47%). These data demonstrate that LH release is pulsatile during pregnancy in the rat, and changes in the characteristics of this secretion occur at different stages of gestation.