The bioavailability of two different forms of medicated feed containing 2000 mg sulfadimidine (SDM) per kg was determined in three groups of eight piglets. In the first group, pharmacokinetic parameters of SDM were determined after a single intravenous dose of 10 mg/kg body weight and after single oral doses of 45 mg/kg body weight ingested either as an oleus solution sprayed directly onto the feed pellets ready for use (SPR) or as a commercially available premix incorporated into the feed before pelletising (PMX). After the single intravenous administration, the mean +/- SD of the volume of distribution was 0.34 +/- 0.05 l/kg, the total body clearance 0.37 +/- 0.07 ml/min.kg, the mean residence time 15.5 +/- 2.5 h, and the elimination half-life 11.1 +/- 2.0 h. Although no statistical significance existed, a single meal with PMX was associated with slightly higher mean values for the maximum serum concentration (Cmax), the time to reach Cmax, and the bioavailability (52.98 +/- 6.60 micrograms/ml, 6.8 +/- 1.1 h, 59.7 +/- 12.1%, respectively, vs. 40.04 +/- 13.19 micrograms/ml, 6.0 +/- 1.4 h, 49.0 +/- 18.6 for SPR). The remaining two groups of piglets received medicated feed with either SPR or PMX during a 3-day period both with restrictive (twice-daily) or ad libitum feeding according to a cross-over design. In all four cases, potentially efficacious plasma SDM concentrations between 50 and 150 micrograms/ml were obtained within 24 h after initiation of the treatment. With PMX, plasma concentrations tended to be higher than with SPR with both feeding regimens. Ad libitum feeding was associated with a significantly higher food intake and hence a higher SDM intake resulting in higher plasma concentrations. Additionally, plasma concentrations were more constant over time with ad libitum feeding whereas they declined considerably between meals in restrictively fed animals. In vitro dissolution tests of the two types of medicated feed revealed that SDM was rapidly released from SPR (58% within 15 min) and that SDM release from PMX was markedly slower (3% within 15 min). Despite the relatively slow rate of in vitro dissolution, in vivo absorption of SDM was satisfactory. It is concluded that both forms of SDM medicated feed may be considered bioequivalent and potentially efficacious in piglets.