The present study exploited the deciduogenic actions of concanavalin A (Con A) in pseudopregnant Quackenbush strain mice to assess whether alterations in uterine Ca2+ status accompany stromal cell transformations that culminate in the decidual cell reaction and the establishment of pregnancy. It was found that the introduction of 15 blastocyst-size Con-A-coated Sepharose beads, but not lectin-free Sepharose beads, into the lumen of the left uterine horn of pseudopregnant mice on day 3 or 4 induced a decidual response that was virtually indistinguishable from that produced by blastocysts during normal pregnancy. The right uterine horn received no treatment and served as a control. Although the simultaneous administration of 45CaCl2 (1 mmol l-1, 200 mCi mmol-1) on day 3 of pseudopregnancy impeded this response, the deciduogenic potential of the Con-A-coated beads was not disrupted when these agents were simultaneously administered on day 4. This experiment with 45CaCl2 produced discrete areas of decidualization, as shown by the pontamine sky blue reaction on day 5 of pseudopregnancy. The decidualized areas contained significantly greater amounts of 45Ca2+ than did the non-decidualized interjacent areas, indicating that the beads preferentially stimulated the rate of influx of the cation into these sites. Much of this cellular 45Ca2+ content was lost in the early afternoon of day 5 of pseudopregnancy at a time when the luminal epithelium undergoes autolytic breakdown.(ABSTRACT TRUNCATED AT 250 WORDS)