A total of 460 Neisseria gonorrhoeae isolates from patients seen at three clinics in Dakar, Sénégal, 1982-1986, have been investigated. In this period a significant change in antimicrobial susceptibility was observed: the percentage of strains susceptible to penicillin (MIC less than or equal to 0.08 micrograms/ml) fell from 61 to 18 (p less than 0.0001) and the percentage of resistant strains (MIC greater than or equal to 1.2 micrograms/ml) increased from 18 to 46. Among penicillin-resistant strains the proportion of penicillinase-producing strains (PPNG) was fairly constant (range 35-55%). The determination of susceptibility to anti-microbial agents performed locally allowed detection of approximately all PPNG strains whereas the increase in the occurrence of strains with chromosomally determined resistance was not revealed. The study comprised 70 PPNG strains of which 19% (13/70) carried the 7.4 kb Asian plasmid and 81% (57/70) the 5.3 kb African plasmid. None of these strains possessed the 38 kb conjugative plasmid, whereas it was found in 4.5% of the 376 non-PPNG strains available for plasmid analysis; 92% (410/446) of all strains had the small 4.2 kb plasmid and 5.4% (24/446) did not contain any plasmid. Overall, auxotype zero and proline-requiring strains were predominant, accounting for 53% (244/460) and 28% (131/460), respectively. In general, PPNG strains carrying the 5.3 kb plasmid were auxotype zero (49/57 = 86%) and those carrying the 7.4 kb plasmid were proline-requiring (9/13 = 69%).