The effect of single infection (2000 infective larvae) with T. spiralis and T. pseudospiralis (both in the intestinal phase) on rat serum methacycline (MC) level was investigated. The infected animals showed a tendency for an initial increase in the serum MC concentration. In rats infected with T. spiralis, as compared with those infected with T. pseudospiralis and with controls, the maximal serum MC concentration was shifted from the 2nd to the 3rd hour after oral MC administration. Rats infected with T. spiralis and T. pseudospiralis, as compared with controls, exhibited a substantial drop in serum MC concentration after 4 h from MC administration. The present results indicate that in intestinal trichinellosis the host intestinal absorption of MC is more rapid and shorter, and the doses of MC absorbed in infected rats are lesser than in controls.