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Preliminary studies on use of the synthetic prostaglandin, fenprostalene, as an abortifacient had indicated that maximum effectiveness was dependent upon slow delivery. Because both route of administration and formulation control delivery rates, the influences of intramuscular (im) vs subcutaneous (sc) injections, and aqueous acetate buffer (AAB) vs polyethylene glycol-400 (PEG) vehicles on the plasma concentration and urinary excretion of fenprostalene were compared. Feedlot heifers were administered 1 mg injections of [13,14-3H]-fenprostalene. Blood samples and total urine excretion were collected during the following 96 h. The maximum concentration of tritium in plasma occurred at 2 h for AAB-im (.90 ng eq/ml), PEG-im (.75 ng eq/ml) and AAB-sc (.64 ng eq/ml), and then declined throughout 24 h with t 1/2 values of 6.1, 9.4 and 9.2 h, respectively. The peak concentration from PEG-sc was lower (.37 ng eq/ml, P less than .05), observed later (4h, P less than .05) and declined more slowly following peak concentration (t 1/2 = 15.1 h, P less than .05). Consistent with delayed absorption, a smaller fraction (P less than .05) of the total radioactivity excreted in urine was recovered during the first 24 h after injection for PEG-sc (85%) than for PEG-im (95%), AAB-sc (97%) or AAB-im (99%). In a tissue distribution study, plasma, urine and fecal samples were collected and heifers were slaughtered at various times following sc injection of 1 mg of [3H] fenprostalene in PEG. Peak concentrations of tritium in plasma occurred between 4 and 8 h and declined with a t 1/2 of 15.2 h.(ABSTRACT TRUNCATED AT 250 WORDS)